New evidence has confirmed that the expansion of the universe is accelerating under the influence of a gravitationally repulsive form of energy that makes up two-thirds of the cosmos
It is an irony of nature that the most abundant form of energy in the universe is also the most mysterious. Since the breakthrough discovery that the cosmic expansion is accelerating, a consistent picture has emerged indicating that two-thirds of the cosmos is made of “dark energy” – some sort of gravitationally repulsive material. But is the evidence strong enough to justify exotic new laws of nature? Or could there be a simpler, astrophysical explanation for the results?
Edge of darkness – data from the Sloan Digital Sky Survey support the existence of dark energy. Credit: Reidar Hahn/Fermilab.
The dark-energy story begins in 1998, when two independent teams of astronomers were searching for distant supernovae, hoping to measure the rate at which the expansion of the universe was slowing down. They were in for a shock: the observations showed that the expansion was speeding up. In fact, the universe started to accelerate long ago, some time in the last 10 billion years.
Like detectives, cosmologists around the world have built up a description of the culprit responsible for the acceleration: it accounts for two-thirds of the cosmic energy density; it is gravitationally repulsive; it does not appear to cluster in galaxies; it was last seen stretching space–time apart; and it goes by the assumed name of “dark energy”. Many theorists already had a suspect in mind: the cosmological constant. It certainly fits the accelerating-expansion scenario. But is the case for dark energy airtight?
The existence of gravitationally repulsive dark energy would have dramatic consequences for fundamental physics. The most conservative suggestions are that the universe is filled with a uniform sea of quantum zero-point energy, or a condensate of new particles that have a mass that is 10-39 times smaller than that of the electron. Some researchers have also suggested changes to Einstein’s general theory of relativity, such as a new long-range force that moderates the strength of gravity. But there are shortcomings with even the leading conservative proposals. For instance, the zero-point energy density would have to be precisely tuned to a value that is an unbelievable factor of 10120 below the theoretical prediction. In view of these extreme solutions, perhaps it is more reasonable to expect a conventional explanation for the accelerating expansion of the universe based on astrophysics (e.g. the effects of dust, or differences between young and old supernovae). This possibility has surely kept more than a few cosmologists awake at night.
Until recently the supernova data were the only direct evidence for the cosmic acceleration, and the only compelling reason to accept dark energy. Precision measurements of the cosmic microwave background (CMB), including data from the Wilkinson Microwave Anisotropy Probe (WMAP), have recently provided circumstantial evidence for dark energy. The same is true of data from two extensive projects charting the large-scale distribution of galaxies – the Two-Degree Field (2DF) and Sloan Digital Sky Survey (SDSS).
Now a second witness has testified. By combining data from WMAP, SDSS and other sources, four independent groups of researchers have reported evidence for a phenomenon known as the integrated Sachs-Wolfe effect. These groups have found that the gravitational repulsion of dark energy has slowed down the collapse of overdense regions of matter in the universe. The case for the existence of dark energy has suddenly become a lot more convincing.
Charting the cosmic expansion
The cosmic expansion, discovered in the late 1920s by Edwin Hubble, is perhaps the single most striking feature of our universe. Not only do astronomical bodies move under the gravitational influence of their neighbours, but the large-scale structure of the universe is being stretched ever larger by the cosmic expansion. A popular analogy is the motion of raisins baking in a very large cake. As the cake rises, the distance between any pair of raisins embedded in the cake grows. If we choose one particular raisin to represent our galaxy, we find that all the other raisins/galaxies are moving away from us in all directions. As a result, our universe has expanded from the hot, dense cosmic soup created in the Big Bang to the much cooler and more rarefied collection of galaxies and clusters of galaxies that we see today.
The light emitted by stars and gas in distant galaxies has likewise been stretched to longer wavelengths during its journey to Earth. This shift in wavelength is given by the redshift, z = (λobs – λ0)/λ0, where λobs is the wavelength we see on Earth and λ0 is the wavelength of the emitted light. For instance, excited hydrogen atoms emit so-called Lyman alpha transition radiation with a characteristic wavelength of λ0 = 121.6 nm when they fall back to the ground state. This transition is seen in distant galaxies, and was used to identify the current record-holder for redshift: a staggering z = 10 galaxy with a Lyman alpha line at λobs = 1337.6 nm (see Physics World April p3). But the redshift describes only the change in the scale of the cosmos, and does not tell us the distance or the age of the universe when the light was actually emitted. If we knew both the distance and the redshift for many objects, we could begin to chart the cosmic expansion.
One of the prime methods for measuring extragalactic distances is to use “standard candles” such as Cepheid variable stars. The luminosity of a Cepheid variable changes periodically with time, with the luminosity being proportional to the period. The distance to a Cepheid can be determined by first measuring its period in order to obtain the luminosity, and then comparing this with the observed intensity to calculate the distance. Thus, redshifts and distances to objects moving in the “Hubble flow” (the region beyond the gravitational influence of our local group of galaxies) have been charted, revealing the Hubble law: d = (cz/H0), where c is the speed of light and H0 = 72 ± 8 km s-1 per megaparsec (Mpc) is the Hubble constant (1 Mpc is equal to 3.26 million light-years).
Before 1998 this linear relationship between distance and redshift had been confirmed for galaxies as far away as about 1000 Mpc, which corresponds to a redshift of 0.24. The extension to higher redshifts was poorly determined, but by making assumptions about the energy density and pressure content of the universe, general relativity can be used to connect redshifts with distances.
However, measuring accurate distances is one of the most difficult tasks in astronomy, and the distance-redshift relationship had not been checked at higher redshifts. Moreover, based on the best information at the time, it was expected that the expansion of the universe should have been slowing down under the attractive influence of gravity – but this had not been confirmed by observations either.
Going the distance
Although Cepheid variable stars have proved extremely valuable as standard candles in astronomy for many years, they are not bright enough to be used at high redshifts. However, astronomers have found a very special type of supernova to take their place.
Type 1a supernovae are the thermonuclear explosions of carbon- and oxygen-rich white dwarfs – stars that are up to 40% more massive than the Sun packed into a radius 100 times smaller. In the early 1930s Subrahmanyan Chandrasekhar showed that white dwarfs can have a maximum mass of 1.4 solar masses. Below this mass, these dense, compact objects are supported against further gravitational collapse by fermion-degeneracy pressure. In other words, the Pauli exclusion principle prevents the tightly packed electrons from occupying the same state. But in a binary system, the strong gravitational field of the white dwarf can pull matter off a companion star until the dwarf “eats” itself to death: the resulting gain in mass destabilizes the star, which then explodes.
Observations of supernovae can be used to chart the history of the cosmic expansion. (a) The distance to a type 1a supernova is readily obtained from its luminosity, which is calibrated by its light curve and spectrum, and its observed intensity. (b) Meanwhile, the expansion of the universe shifts features in the supernova spectrum to longer wavelengths by a factor characterized by the redshift. (c) By plotting distance versus redshift for a large number of supernovae, we can chart how the universe has expanded over time. The orange circles are data points with the error bars omitted for clarity (see Riess et al. in further reading), along with the favoured theoretical prediction: a universe with 30% matter and 70% cosmological constant (blue). Also shown are predictions for a universe with 30% matter and spatial curvature (red dashed), and for 100% matter (purple dashed). The difference between acceleration and deceleration is revealed where the theoretical curves start to diverge. The transition from deceleration to acceleration is more subtle: the green line shows a coasting cosmos that is neither accelerating nor decelerating. The expansion speeds up close to where the data reach their maximum deviation from this curve (near z = 0.5). Hubble’s view of the cosmic expansion was limited to objects at distances less than a few megaparsecs (i.e. a small region on the left of the figure).
Serendipitously, the luminosity of the exploding white dwarf is very nearly a standard candle. In the mid-1990s this prompted two teams of astronomers – the High-z Supernova Search Team and the Supernova Cosmology Project – to begin observational campaigns to measure the distances and redshifts of type 1a supernovae, in the hope of confirming that the cosmic expansion was indeed slowing down as expected. The results, based on some 100 or so supernovae extending out to a redshift of about 1, were stunning. The two teams found that high-z supernovae are fainter – and therefore more distant – than they should be in a decelerating universe. The researchers had discovered that the expansion of the universe is accelerating (figure 1).
Not surprisingly, interest in supernovae has grown tremendously since then. The Hubble Space Telescope and the premier ground-based observing facilities are chasing the rise and fall of light from supernovae, while smaller telescopes are making surveys and studying nearby events. So far, distances to more than 300 type 1a supernovae have been obtained, and data for many more are currently being analysed. With systematic effects coming under control (see “Focus on supernovae” in Further information), it now appears that the universe started to accelerate as recently as between about five and seven billion years ago (see Riess et al. in further reading). Theorists have been just as busy as observers, trying to unravel what is behind the accelerating expansion.
The missing energy
The supernova observations call out for some gravitationally repulsive substance to drive the cosmic acceleration. Astronomers have long been aware of a missing-energy problem: the luminous mass of galaxies and clusters falls far short of the gravitational mass. This difference is attributed to the presence of dark matter – a cold, non-relativistic material most likely in the form of exotic particles that interact very weakly with atoms and light.
However, observations suggest that the total amount of matter in the universe – including all the dark matter – accounts for just one-third of the total energy. This has been confirmed by surveys such as the 2DF and SDSS projects, which have mapped the positions and motions of millions of galaxies. But general relativity predicts that there is a precise connection between the expansion and the energy content of the universe. We therefore know that the collective energy density of all the photons, atoms, dark matter and everything else ought to add up to a certain critical value determined by the Hubble constant: ρcritical = 3H02/8π G, where G is the gravitational constant. The snag is that they do not.
Mass, energy and the curvature of space-time are intimately related in relativity. One explanation is therefore that the gap between the critical density and the actual matter density is filled by the equivalent energy density of a large-scale warping of space that is discernable only on scales approaching c/H0 (about 4000 Mpc).
Fortunately, the curvature of the universe can be determined by making accurate, precision measurements of the cosmic microwave background (CMB). A relic from some 400,000 years after the Big Bang, the CMB is black-body radiation from the primordial plasma. As the universe cooled below about 3000 K the plasma became transparent to photons, allowing them to propagate freely through space. Today, almost 15 billion years later, we see a thermal bath of photons at a temperature of 2.726 K that are redshifted to the microwave region of the spectrum by the cosmic expansion (see “The cosmic microwave background”).
The remarkable images of the CMB captured by the WMAP satellite show slight variations in the photon temperatures across the sky – known as the CMB anisotropy – reflecting slight variations in the density and motion of the early universe. These variations, which occur at the level of a few parts per 100,000, reveal the blueprint for the large-scale structure of galaxies and clusters that we see today.
The coldest/hottest spots in the CMB are due to photons that climbed out of the gravitational potentials of the largest over/under dense regions, and the size of these regions is well determined by the physics of the plasma. When viewed across the entire universe, the apparent angular size of these anisotropies would be about 0.5º if the universe has enough warping to fill the energy density gap, and twice as large in the absence of any warping. The easiest way to picture this geometric effect is to imagine a triangle with a fixed base and legs drawn on surfaces with different curvatures: for a saddle surface/sphere the interior angles are all smaller/larger than for the same triangle drawn on a flat surface with planar or Euclidean geometry.
Since 1999 a sequence of experiments – TOCO, MAXIMA, BOOMERANG and most recently WMAP – has confirmed that the CMB spots are about 1° across: the large-scale geometry of the universe is “flat”. For the missing-energy problem, this means something other than curvature must be responsible for the energy density gap.
To some cosmologists, this result felt like a case of déjà vu (see “A brief history of dark energy” in Further information). Inflation, the best theory around for the origin of fluctuations in the CMB, proposes that the very early universe underwent a period of accelerated expansion, which was driven by a particle called the inflaton. However, inflation would have stretched away any large-scale spatial curvature, leaving the geometry of the universe Euclidean or flat. The evidence therefore suggests a form of energy that does not cluster in galaxies, that is gravitationally repulsive, and that might possibly be due to some new particle not unlike the inflaton.
Convincing as the CMB data were, the only direct evidence for cosmic acceleration – that is for gravitationally repulsive dark energy – came from the supernova data. But things are beginning to change. By combining the precision measurements of the CMB by WMAP with radio, optical and X-ray probes of the large-scale distribution of matter, astrophysicists have also teased out further evidence that the expansion rate is quickening. It appears that the gravitational potential wells of dense and overdense regions in the universe have been stretched and made shallower over time, as if under the influence of repulsive gravity.
This phenomenon is known as the integrated Sachs-Wolfe (ISW) effect, and it leads to a correlation between the temperature anisotropies in the CMB and the large-scale structure of the universe. Although the primordial plasma became transparent to photons after the universe cooled, the photons did not travel unhindered afterwards. The cosmos is riddled with inhomogeneities that are strong on small length scales (where matter has clumped to form stars, nebulae and galaxies), and progressively weaker on larger length scales, where galaxies and clusters ride on gentle waves in the matter density. On their flight paths, photons fall into and climb out of the corresponding gravitational potentials.
Dark energy influences cosmic microwave background (CMB) photons, in particular via the integrated Sachs-Wolfe effect (ISW). CMB photons zipping across the universe gain energy by falling into gravitational potential wells, and lose energy when they climb out again (top trajectory). For the shallow potentials on large scales, which correspond to over- and under- dense regions extending across hundreds of megaparsecs, the overall loss and gain of energy cancel. But this is only true in a universe in which the full critical energy density comes from atoms and dark matter. In the presence of dark energy, however, the ISW effect comes in to play: the expansion of the universe is fast enough to stretch the potentials and make them shallower, which means that a photon falling into an overdense region gains more energy than it loses when climbing out (bottom trajectory). Regions of space in which matter has clustered should therefore correspond to hotter CMB photons, whereas underdense regions should lead to colder CMB photons. By comparing the CMB and the large-scale structure of the universe at different wavelengths, four independent groups of cosmologists have found signs of the ISW effect, providing a new line of evidence that is consistent with cosmic acceleration driven by dark energy.
When the cosmic radiation was first detected almost 40 years ago, Rainer Sachs and Art Wolfe showed that a time-varying potential would impart an energy shift to CMB photons that passed by (figure 2). A photon gains energy when it falls into the gravitational potential of an overdense region, and expends energy when it climbs back out. If the potential has deepened over the course of this process, the photon therefore loses energy overall. If the potential becomes shallower over time, the photon gains energy.
In a universe where the full critical energy density comes from atoms and dark matter only, the weak gravitational potentials on very long length scales – which correspond to gentle waves in the matter density – evolve too slowly to leave a noticeable imprint on the CMB photons. These overdense regions simply accrete the surrounding matter at the same rate at which the cosmic expansion stretches the waves longer, leaving the potentials unchanged. However, under the faster expansion rate of a universe that contains dark energy, the accretion of matter cannot keep up with the stretching. In effect, gravitational collapse is slowed by the repulsive dark energy. Consequently, gravitational potentials grow shallower and photons gain energy as they pass by. Similarly, photons lose energy passing through underdense regions.
It turns out that the large-scale gravitational potentials experienced by the CMB photons correspond to the same overdense/underdense regions seen in the mega-galaxy sky surveys at various wavelengths. CMB photons coming from the same region where galaxies cluster are boosted slightly hotter by the ISW effect. Hence, there should be a positive correlation between the CMB temperature and large-scale structure patterns on the sky. Now, nearly five years after the first supernova results, four independent groups have announced the first detections of this ISW effect.
By comparing the temperature-anisotropy pattern of the cosmic microwave background, measured by the WMAP satellite (top), with the intensity variations in the hard X-ray background, obtained via the HEAO-1 mission (bottom), Stephen Boughn and Robert Crittenden have identified evidence for the integrated Sachs-Wolfe effect (ISW). Three other groups have also found preliminary signals of the ISW effect in comparisons of WMAP data with optical and radio maps that trace the distribution of matter.
Stephen Boughn of Haverford College and Robert Crittenden of the University of Portsmouth have found correlations between the WMAP data and two probes of large-scale structure: radio data from the NRAO/VLA Sky Survey (NVSS) and measurements of the hard X-ray background made by HEAO-1, a satellite launched in 1977 (figure 3). The WMAP team has also seen correlations between its data and the NVSS results. Moreover, the Sloan Digital Sky Survey team, along with Pablo Fosalba of the Institut d’Astrophysique de Paris and co-workers, has found evidence for the ISW effect when comparing the WMAP and SDSS data sets (see further reading).
Although the ISW evidence on its own is not yet strong enough to discriminate between expansion caused by spatial curvature and expansion caused by dark energy, when combined with the CMB data for a flat universe the weight of the evidence tilts in favour of dark energy. Taken together, the results are tantalizing. Furthermore, the ISW evidence probes the effects of dark energy on distances down to about 100 Mpc, which is a completely different scale than that of supernovae. This provides a new and independent line of evidence for the effects of dark energy.
The biggest mystery of the cosmic acceleration is not that it suggests that two-thirds of the universe is made of stuff that we cannot see, but that it suggests the existence of a substance that is gravitationally repulsive. To examine this strange property of dark energy it is helpful to introduce a quantity w = pdark/ρdark, where pdark is the mean pressure and ρdark is the density of dark energy in the universe. This new quantity is similar to the equation of state for a gas.
In general relativity the rate of change in the cosmic expansion is proportional to -(ρtotal + 3ptotal), where ρtotal is the density of all the matter and energy in the universe and ptotal is the corresponding pressure. To account for the accelerated expansion, however, this quantity must be positive. Since ρtotal is a positive quantity, and the mean pressure due to both ordinary and dark matter is negligible because it is cold or non-relativistic, we arrive at the requirement that 3w x ρdark + ρtotal < 0 for an accelerating expansion. Since ρdark ~ 2/3ρtotal, we find that w≤-1/2, so the pressure of the dark energy is not just a little negative but a lot negative!
Why does pressure influence the expansion of the universe? Einstein showed that matter and energy curve space-time. So for a hot gas the kinetic motions of atoms contribute to their gravitational pull, as measured through the acceleration of distant test bodies. However, the forces required to contain or isolate the hot gas count against this pressure bonus. The universe, on the other hand, is neither isolated nor bounded. The expansion of a cosmos filled by hot gases is effectively slowed by the attraction of its self-gravity, more so than a universe that is filled with an equivalent energy density of cold, pressureless gas. And by the same logic a medium that allows negative pressure such that ρtotal + 3ptotal < 0 will expand more quickly, repelled by its own anti-gravity.
Negative pressure is not such a rare phenomenon. The water pressure in certain tall trees becomes negative as nourishment is pulled up through their vascular system, and the pressure tangential to a uniform electric or magnetic field is also negative. In these cases, the pressure is somewhat like a stretched spring under tension, exerting an inward force. On the microscopic level, a bath of Higgs bosons – the hypothetical particles that give rise to mass in the Standard Model of particle physics – exerts negative pressure when its thermal or kinetic excitations are small. Indeed, the inflaton can be viewed as a heavier version of the Higgs, and one of the proposed forms of dark energy called quintessence might be an even lighter version of the Higgs (see “Dark energy: the suspects” in Further information).
In principle there is no lower bound to the pressure in the universe, although strange things happen if w falls below -1 (an isolated lump of such material could appear to have negative mass, which is just what one might need to prop open a wormhole). However, most proposed forms of dark energy can buckle or bend only slightly, and even then only over distances much bigger than galaxies, making it hard to get a handle on the stuff. But one thing is certain: such strongly negative pressure does not happen for normal particles and fields in general relativity.
A major challenge in cosmology is to determine the amount of dark energy, expressed as a fraction of the critical density by Ωdark = ρdark/ρcritical, and its degree of gravitational repulsion, which is characterized by the quantity w (see text). In a spatially flat universe Ωdark = 1 – Ωm, where Ωm is the amount of matter as a fraction of the critical density. Supernova data (red region) suggest that matter – both ordinary and dark matter – accounts for less than 50% of the total or critical density of the universe, and possibly as little as 20%. Since the relationship between distance and redshift depends on the repulsive strength of the dark energy (see figure 1), the supernova data also suggest that w is less than about -0.8. Measurements of the cosmic microwave background also place limits on these two numbers (blue region): the apparent angular sizes of features in the temperature-anisotropy pattern likewise depend on the distance-redshift relation, although at a much higher redshift of z ~ 1100. The overlap of the two sets of results suggest that dark energy accounts for between 62% and 76% of the total critical energy density, and that the value of w lies between -1.3 and -0.9.
The detailed observations lead to slightly tighter constraints on the dark-energy parameters than those given by the simple estimates above. When the predictions of the different theoretical models are combined with the best measurements of the cosmic microwave background, galaxy clustering and supernova distances, we find that 0.62 < Ωdark < 0.76, where Ωdark = ρdark/ρcritical, and -1.3 < w < -0.9 (figure 4).
Looking ahead, darkly
The evidence for gravitationally repulsive dark energy is strong, but there are gaps in our knowledge. The physics of type 1a supernovae is not fully understood, dark matter is still on the loose, and there are a few unexpected features in the CMB spectrum that we do not yet fully understand. While some of these do not seem to be related to the cosmic acceleration, the entire scenario must fit together in order to be compelling. The good news is that we can expect lots of new data. WMAP and a host of balloon-borne and ground-based experiments are continuing to scour the CMB sky, with the Planck satellite due to follow later this decade. New techniques are also being developed to extract information about dark energy, such as plans to study the evolution of the abundance of clusters of galaxies. Another, more ambitious method proposes to infer the ISW effect at different vantage points and redshifts in the universe.
Supernova studies will receive a huge boost if the Joint Dark Energy Mission (JDEM) being proposed by the US Department of Energy and NASA goes ahead. Although the launch is about 10 years away, this dedicated satellite telescope will deliver the final word on cosmic acceleration from supernovae. JDEM also promises an extensive weak-lensing survey that will blaze a new trail towards understanding the nature of dark energy through its influence on cosmic structures and evolution. Naturally, healthy competition with ground-based observers will keep the intervening years exciting.
The aim of all this activity is, of course, to answer the question, what is the dark energy? If w is about -1, then a cosmological constant might be the solution. If w is more than -1, the right answer might be quintessence. And we cannot rule out a new twist to gravity that even Einstein did not foresee: while most theories that link gravitational and quantum physics predict novel behaviour on microscopic length scales or at very early times in the universe, few, if any, anticipate new effects on the largest length scales in the present day. And what if w is less than -1? Whatever the answer, something mysterious is at work in the cosmos.
Focus on supernovae
How can we be certain that the flux of light from supernovae is indeed diluted and dimmed due to travelling the greater distances that result from the accelerated expansion of the universe? Perhaps the supernovae are closer than we suspect and other effects are at work. The enormous implications of cosmic acceleration have brought great scrutiny to the astrophysics of type 1a supernovae.
It should be stressed that type 1a supernovae are not quite standard candles. However, the luminosity can be standardized: detailed observations of nearby supernovae at known distances have revealed a pattern that can be used to calibrate the luminosity using the light curve and spectrum. But it is possible that this technique may not be valid for more distant supernovae formed much earlier in the history of the universe. For instance, the star-forming environment is expected to evolve over time as the birth and death of stars pollutes the stellar nursery with metals. Could these changes in environment translate into changes in the properties of white dwarfs and supernova explosions? Are distant supernovae fainter simply because they are fainter? However, astrophysicists have found no such link between environment and luminosity.
Finally, there is always the possibility that our view is obscured by cosmic dust. If so, ever more distant supernovae would appear dimmed, giving the illusion of an eternally accelerating universe. However, high-redshift supernovae do not show this trend. In fact, recent results give evidence for past deceleration.
A brief history of dark energy
Dark energy, or something like it, has made numerous appearances in cosmology. Einstein initially introduced a cosmological constant, Λ, in constructing the first cosmological model in his nascent gravitational theory. The cosmic expansion had not yet been discovered, and his calculations correctly indicated that a universe containing matter could not be held static without the mathematical addition of -Λ. The effect was equivalent to filling the universe with a pristine sea of negative energy, upon which stars and nebulae drift. The later discovery of the expansion obviated the need for such an ad hoc addition to his theory.
In the following decades, desperate theorists periodically recycled the cosmological constant in an effort to explain new astronomical phenomena. These resurgences were always short-lived, after closer inspection or subsequent observations revealed more reasonable explanations for the data. Yet developments in particle physics in the late 1960s suggested that the vacuum energy of all particles and fields should inevitably generate a term like Λ. Moreover, a phase transition in the first few seconds after the Big Bang might have left the cosmos filled by a cosmological constant.
In 1980 the theory of inflation was developed: in this theory the early universe undergoes a brief period of accelerated, exponential expansion, with the negative pressure that drives the expansion coming from a new particle called the inflaton, rather than Λ. Inflation has been wildly successful. It resolves various paradoxes associated with the Big Bang model, such as the horizon and flatness problems, and its predictions are consistent with measurements of large-scale structure and the cosmic microwave background.
Inflation also predicts that a characteristic pattern of long-wavelength gravitational waves would have been created in the early universe. These waves are literally gravitons – the hypothetical particles that carry the gravitational force – that have been stretched to macroscopic lengths by the cosmic expansion. The detection of these waves would provide a unique signature of inflation.
Dark energy: the suspects
• Cosmological constant (w = -1)
Originally introduced by Albert Einstein, it was later suggested by Yakov Zel’dovich that quantum vacuum energy would produce a constant energy density and pressure. However, theoretical predictions yield a cosmological constant that is 120 orders of magnitude higher than the observational value. Regardless of cosmology, quantum vacuum energy exists. Whether the cosmic contribution is in fact zero, or finely tuned, is one of the outstanding challenges in physics.
• Quintessence (w > -1)
A form of energy with negative pressure that varies with space and time. Quintessence is dynamic, unlike the cosmological constant, and its average energy density and pressure slowly decay with time. This feature might help to explain the tuning and sudden onset of cosmic acceleration. Modelled as a scalar field, quintessence predicts particle-like excitations with a mass of about 10-33 eV (see Caldwell and Steinhardt in further reading).
• Other vacuum energy (w < -1)
Unless we are the victims of a conspiracy of systematic effects, w < -1 is the sign of really exotic physics. In one model, quantum effects of a quintessence-like field lead to modifications of general relativity, while other models suggest that the dark-energy density actually grows with time, possibly causing the universe to end in a catastrophic “big rip”. Other novel ideas include an exotic field that causes a cosmological-constant-like acceleration but that varies in space.
Modification of general relativity
Various attempts have been made to modify Einstein’s general theory of relativity, and therefore avoid the need for exotic matter to drive the accelerated expansion. While some are difficult to distinguish from quintessence, many predict violations of the equivalence principle (which is the bedrock of general relativity) or departures from the universal 1/r gravitational potential.
About the author
Robert R Caldwell is in the Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755-3528, US, e-mail firstname.lastname@example.org
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In an article titled “The Cosmic Triangle: Revealing the State of the Universe,” which appears in the May 28, 1999 issue of the journal Science, a group of cosmologists and physicists from Princeton University and Lawrence Berkeley National Laboratory survey the wide range of evidence which, they write, “is forcing us to consider the possibility that some cosmic dark energy exists that opposes the self-attraction of matter and causes the expansion of the universe to accelerate.”
Dark energy is hardly science fiction, although no less intriguing and full of mystery for being real science.
“The universe is made mostly of dark matter and dark energy,” says Saul Perlmutter, leader of the Supernova Cosmology Project headquartered at Berkeley Lab, “and we don’t know what either of them is.” He credits University of Chicago cosmologist Michael Turner with coining the phrase “dark energy” in an article they wrote together with Martin White of the University of Illinois for Physical Review Letters.
In the May 28 Science article, Perlmutter and Neta Bahcall, Jeremiah Ostriker, and Paul Steinhardt of Princeton use the concept of dark energy in discussing their graphic approach to understanding the past, present, and future status of the universe. The Cosmic Triangle is the authors’ way of presenting the major questions cosmology must answer: “How much matter is in the universe? Is the expansion rate slowing down or speeding up? And, is the universe flat?”
The possible answers are values for three terms in an equation that describes the evolution of the universe according to the general theory of relativity. By plotting the best experimental observations and estimates within the triangle, scientists can make preliminary choices among competing models.
The mass density of the universe is estimated by deriving the ratio of visible light to mass in large systems such as clusters of galaxies, and in several other ways. For several decades the evidence has been building that mass density is low and that most of the matter in the universe is dark.
Changes in expansion rate are estimated by comparing the redshifts of distant galaxies with the apparent brightness of Type 1a supernovae found in them. These measurements suggest that the expansion of the universe is accelerating.
Curvature is estimated from measurements of the anisotropy (temperature fluctuation) of the cosmic microwave background radiation (CMB), a remnant of the Big Bang. Although uncertainty is large, current results suggest a flat universe.
The Cosmic Triangle eliminates some popular models, such as a high-density universe that is slowing down and will eventually recollapse, as well as a nearly empty universe with no dark energy and low mass. While the evidence from galactic clusters shows that mass density is low, supernova evidence for acceleration shows that dark energy must be abundant.
“These two legs of the Cosmic Triangle agree with the evidence from the CMB that the universe is flat,” Perlmutter says, adding that “this is a remarkable agreement for these early days of empirical cosmology.”
Thus the Cosmic Triangle suggests that the standard inflationary scenario is on the right track: one of its key predictions is a flat universe.
Various types of dark energy have been proposed, including a cosmic field associated with inflation; a different, low-energy field dubbed “quintessence”; and the cosmological constant, or vacuum energy of empty space. Unlike Einstein’s famous fudge factor, the cosmological constant in its present incarnation doesn’t delicately (and artificially) balance gravity in order to maintain a static universe; instead, it has “negative pressure” that causes expansion to accelerate.
“The term Cosmic Triangle sounds kind of New Agey,” says Perlmutter, “but it’s a good way to portray the quantities in these comparisons, and it’s fun for people who like to plot the possibilities” — an evolving task that, among other choices, will require finding an answer to “the most provocative and profound” issue of all, the nature of cosmic dark energy.
The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.
The discovery in 1998 that the Universe is actually speeding up its expansion was a total shock to astronomers. It just seems so counter-intuitive, so against common sense. But the evidence has become convincing.
The evidence came from studying distant type Ia supernovae. This type of supernova results from a white dwarf
star in binary system. Matter transfers from the normal star to the white dwarf until the white dwarf attains a critical mass (the Chandrasekhar limit) and undergoes a thermonuclear explosion. Because all white dwarfs acheive the same mass before exploding, they all achieve the same luminosity and can be used by astronomers as “standard candles.” Thus by observing their apparent brightness, astronomers can determine their distance using the 1/r2 law.
By knowing the distance to the supernova, we know how long ago it occurred. In addition, the light from the supernova has been red-shifted by the expansion of the unviverse. By measuring this redshift from the spectrum of the supernova, astronomers can determine how much the universe has expanded since the explosion. By studying many supernovae at different distances, astronomers can piece together a history of the expansion of the universe.
In the 1990’s two teams of astronomers, the Supernova Cosmology Project and the High-Z Supernova Search, were looking for distant type Ia supernovae in order to measure the expansion rate of the universe with time. They expected that the expansion would be slowing, which would be indicated by the supernovae being brighter than their redshifts would indicate. Instead, they found the supernovae to be fainter than expected. Hence, the expansion of the universe was accelerating!
In addition, measurements of the cosmic microwave background indicate that the universe has a flat geometry on large scales. Because there is not enough matter in the universe – either ordinary or dark matter – to produce this flatness, the difference must be attributed to a “dark energy”. This same dark energy causes the acceleration of the expansion of the universe. In addition, the effect of dark energy seems to vary, with the expansion of the Universe slowing down and speeding up over different times.
Astronomers know dark matter is there by its gravitational effect on the matter that we see and there are ideas about the kinds of particles it must be made of. By contrast, dark energy remains a complete mystery. The name “dark energy” refers to the fact that some kind of “stuff” must fill the vast reaches of mostly empty space in the Universe in order to be able to make space accelerate in its expansion. In this sense, it is a “field” just like an electric field or a magnetic field, both of which are produced by electromagnetic energy. But this analogy can only be taken so far because we can readily observe electromagnetic energy via the particle that carries it, the photon.
Some astronomers identify dark energy with Einstein’s
Cosmological Constant. Einstein introduced this constant into his general relativity when he saw that his theory was predicting an expanding universe, which was contrary to the evidence for a static universe that he and other physicists had in the early 20th century. This constant balanced the expansion and made the universe static. With Edwin Hubble’s discovery of the expansion of the Universe, Einstein dismissed his constant. It later became identified with what quantum theory calls the energy of the vacuum.
In the context of dark energy, the cosmological constant is a reservoir which stores energy. Its energy scales as the universe expands. Applied to the supernova data, it would distinguish effects due to the matter in the universe from those due to the dark energy. Unfortunately, the amount of this stored energy required is far more than observed, and would result in very rapid acceleration (so much so that the stars and galaxies would not form). Physicists have suggested a new type of matter, “quintessence,” which would fill the universe like a fluid which has a negative gravitational mass. However, new constraints imposed on cosmological parameters by Hubble Space Telescope data rule out at least simple models of quintessence.
Other possibilities being explored are topological defects, time varying forms of dark energy, or a dark energy that does not scale uniformly with the expansion of the universe
Dark Energy, Dark Matter
In the early 1990’s, one thing was fairly certain about the expansion of the Universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the Universe had to slow. The Universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the Universe was actually expanding more slowly than it is today. So the expansion of the Universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.
Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein’s theory of gravity, one that contained what was called a “cosmological constant.” Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein’s theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don’t know what the correct explanation is, but they have given the solution a name. It is called dark energy.
What Is Dark Energy?
More is unknown than is known. We know how much dark energy there is because we know how it affects the Universe’s expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 70% of the Universe is dark energy. Dark matter makes up about 25%. The rest – everything on Earth, everything ever observed with all of our instruments, all normal matter – adds up to less than 5% of the Universe. Come to think of it, maybe it shouldn’t be called “normal” matter at all, since it is such a small fraction of the Universe.
This diagram shows changes in the rate of expansion since the Universe’s birth 14 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the Universe began flying apart at a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark energy that is pulling galaxies apart. Credit: NASA/STSci/Ann Feild
One explanation for dark energy is that it is a property of space. Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein’s gravity theory, the version that contains a cosmological constant, makes a second prediction: “empty space” can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the Universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the Universe. Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, “empty space” is actually full of temporary (“virtual”) particles that continually form and then disappear. But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong – wrong by a lot. The number came out 10120 times too big. That’s a 1 with 120 zeros after it. It’s hard to get an answer that bad. So the mystery continues.
Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the Universe is the opposite of that of matter and normal energy. Some theorists have named this “quintessence,” after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don’t know what it is like, what it interacts with, or why it exists. So the mystery continues.
A last possibility is that Einstein’s theory of gravity is not correct. That would not only affect the expansion of the Universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein’s theory is known to do, and still give us the different prediction for the Universe that we need? There are candidate theories, but none are compelling. So the mystery continues.
The thing that is needed to decide between dark energy possibilities – a property of space, a new dynamic fluid, or a new theory of gravity – is more data, better data. The Joint Dark Energy Mission (JDEM) is a NASA mission in the planning stages, being developed jointly by NASA and the Department of Energy. Its goal will be to provide observations of the Universe that will allow theorists to discriminate between theories and, perhaps, finally lead to the solution of the mystery.
Universe Dark Energy-2 NGC 4555 Photo: NGC 4555 – this large, isolated, elliptical galaxy is embedded in a cloud of 10-million-degree Celsius gas. NASA/CXC/E.O’Sullivan et al. This image is not a picture of dark matter. It is a picture of its effects, captured by the Chandra X-ray Observatory. It’s a galaxy surrounded by a cloud of extremely hot gas. In order for the gas to stay around the galaxy, a halo of matter we cannot see must be holding it there with its gravity.
By fitting a theoretical model of the composition of the Universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~70% dark energy, ~25% dark matter, ~5% normal matter. What is dark matter?
We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the Universe to make up the 25% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.
However, at this point, there are still a few dark matter possibilities that are viable. Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as massive compact halo objects, or “MACHOs“. But the most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS (Weakly Interacting Massive Particles).
Credit to : Various Website
This article is a collection or compilation ; when I was wandering in WWW…looking for Dark Energy.