Dark Energy, Dark Matter
In the early 1990s, 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?
This diagram reveals changes in the rate of expansion since the universe's birth 15 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 as a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart.
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 68% of the Universe is dark energy. Dark matter makes up about 27%. 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.
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.
This image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520. The result could present a challenge to basic theories of dark matter.
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.
What Is Dark Matter?
One of the most complicated and dramatic collisions between galaxy clusters ever seen is captured in this new composite image of Abell 2744. The blue shows a map of the total mass concentration (mostly dark matter).
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, ~68% dark energy, ~27% 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 27% 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).
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Roughly 80 percent of the mass of the universe is made up of material that scientists cannot directly observe. Known as dark matter, this bizarre ingredient does not emit light or energy. So why do scientists think it dominates?
Studies of other galaxies in the 1950s first indicated that the universe contained more matter than seen by the naked eye. Support for dark matter has grown, and although no solid direct evidence of dark matter has been detected, there have been strong possibilities in recent years.
The familiar material of the universe, known as baryonic matter, is composed of protons, neutrons and electrons. Dark matter may be made of baryonic or non-baryonic matter. To hold the elements of the universe together, dark matter must make up approximately 80 percent of its matter.
The missing matter could simply be more challenging to detect, made up of regular, baryonic matter. Potential candidates include dim brown dwarfs, white dwarfs and neutrino stars. Supermassive black holes could also be part of the difference. But these hard-to-spot objects would have to play a more dominant role than scientists have observed to make up the missing mass, while other elements suggest that dark matter is more exotic.
Most scientists think that dark matter is composed of non-baryonic matter. The lead candidate, WIMPS(weakly interacting massive particles), have ten to a hundred times the mass of a proton, but their weak interactions with "normal" matter make them difficult to detect. Neutralinos, massive hypothetical particles heavier and slower than neutrinos, are the foremost candidate, though they have yet to be spotted. The smaller neutral axion and the uncharched photinos are also potential placeholders for dark matter.
A third possibility exists — that the laws of gravity that have thus far successfully described the motion of objects within the solar system require revision.
Proving the unseen
If scientists can't see dark matter, how do they know it exists?
Scientists calculate the mass of large objects in space by studying their motion. Astronomers examining spiral galaxies in the 1950s expected to see material in the center moving faster than on the outer edges. Instead, they found the stars in both locations traveled at the same velocity, indicating the galaxies contained more mass than could be seen. Studies of the gas within elliptical galaxies also indicated a need for more mass than found in visible objects. Clusters of galaxies would fly apart if the only mass they contained were visible to conventional astronomical measurements.
Albert Einstein showed that massive objects in the universe bend and distort light, allowing them to be used as lenses. By studying how light is distorted by galaxy clusters, astronomers have been able to create a map of dark matter in the universe.
All of these methods provide a strong indication that the most of the matter in the universe is something yet unseen.
Dark matter versus dark energy
Although dark matter makes up most of the matter of the universe, it only makes up about a quarter of the composition. The universe is dominated by dark energy.
After the Big Bang, the universe began expanding outward. Scientists once thought that it would eventually run out of the energy, slowing down as gravity pulled the objects inside it together. But studies of distant supernovae revealed that the universe today is expanding faster than it was in the past, not slower, indicating that the expansion is accelerating. This would only be possible if the universe contained enough energy to overcome gravity — dark energy.
Dark matter is a hypothetical type of matter composing the approximately 27% of the mass and energy in the observable universe that is not accounted for by dark energy, baryonic matter, and neutrinos. The name refers to the fact that it does not emit or interact with electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum. Although dark matter cannot be directly observed with conventional electromagnetic telescopes, its existence and properties are inferred from its various gravitational effects such as the motions of visible matter, via gravitational lensing, its influence on the universe's large-scale structure, and its effects in the cosmic microwave background. Dark matter is transparent to electromagnetic radiation and/or is so dense and small that it fails to absorb or emit enough radiation to be detectable with current imaging technology.
Estimates of masses for galaxies and larger structures via dynamical and general relativistic means are much greater than those based on the mass of the visible "luminous" matter.
The standard model of cosmology indicates that the total mass–energy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. Thus, dark matter constitutes 84.5% of total mass, while dark energy plus dark matter constitute 95.1% of total mass–energy content. The great majority of ordinary matter in the universe is also unseen, since visible stars and gas inside galaxies and clusters account for less than 10% of the ordinary matter contribution to the mass-energy density of the universe.
The dark matter hypothesis plays a central role in current modeling of cosmic structure formation and galaxy formation and evolution and on explanations of the anisotropies observed in the cosmic microwave background (CMB). All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which is observable via electromagnetic signals.
The most widely accepted hypothesis on the form for dark matter is that it is composed of weakly interacting massive particles (WIMPs) that interact only through gravity and the weak force.
Although the existence of dark matter is generally accepted by most of the astronomical community, a minority of astronomers argue for various modifications of the standard laws of general relativity, such as MOND, TeVeS, and Conformal gravity that attempt to account for the observations without invoking additional matter.
Many experiments to detect proposed dark matter particles through non-gravitational means are under way.
What is Dark Matter?
In the past few decades, cosmologists have discovered that 'regular' matter – the stuff we can see and that makes up stars, planets, rocks, gas clouds and dust – only accounts for a small fraction of the total mass in our Universe. Scientists call this 'regular' matter baryonic matter, so called because it is made up of particles called baryons. Dark matter is the name we give to matter we cannot observe directly, and that appears to be made up of something other than baryons.
The first evidence for the existence of dark matter emerged in the 1930s when an astronomer called Fritz Zwicky observed the Coma cluster – a massive cluster of over 1000 galaxies, all gravitationally bound to one another. Zwicky looked at how many galaxies were visible in the Coma cluster and made an estimate of the total amount of matter in the cluster based on the mass of an average galaxy. He also measured the velocities of some of the galaxies in the cluster, and deduced that many of them were moving very fast – so fast, in fact, that they should have been able to escape the gravitational pull of the other galaxies in the cluster and escape into deep space. This implied that the cluster was much more massive than Zwicky had calculated; the most natural way to account for this was to assume that much of the cluster's mass was invisible.
Since Zwicky's early observations, similar data from other galaxy clusters has yielded the same result – we consistently see that clusters of galaxies appear to have masses tens of times larger than their luminous matter content can account for. Further evidence for dark matter has since been discovered in individual galaxies themselves. In the 1970s, astronomers noticed that the stars in the outer parts of several nearby galaxies were orbiting their galactic centres faster than expected, and were apparently moving fast enough to escape their host galaxies. This again implies that the mass of the galaxy is much higher than can be accounted for by the visible stars and gas alone. From this, astronomers concluded that dark matter appears to surround galaxies in a halo, extending far beyond the edges of the visible galaxies themselves, as well as existing in the space between galaxies in clusters.
The most interesting thing about dark matter is not simply that we can't see it, it's that we know dark matter is not made of the same stuff as normal baryonic matter. This is actually why we can't see it – baryons interact with each other through gravity, nuclear forces and the electrostatic force. These interactions are what allow baryonic matter (such as stars) to emit light, and what prevent you from putting your hand through a table – the particles of your hand are electrostatically repelled from the particles in the table. Dark matter, however, only interacts through gravity. This is why we see its effects on the motions of galaxies and stars, but why we can't see it directly; it does not emit or absorb light. Dark matter particles can also pass through regular matter almost completely undetected since they don't interact electrostatically, meaning we can't touch it or sense it in any direct way.
One of the reasons astronomers believe dark matter is non-baryonic in nature is because it is possible to calculate how much baryonic matter there actually is in the Universe. By measuring the ratio of hydrogen, the most common element in the Universe, and its heavier isotope deuterium, astronomers have been able to work out how much baryonic matter there must be. This is because deuterium is very difficult to produce, and almost all the deuterium in existence today was formed in the Big Bang. The exact amount of baryonic matter created influenced the hydrogen/deuterium ratio, and as a result we now know that only 4% of the mass-energy content of the Universe is in the form of baryonic matter. We know from other observations that all the matter in the Universe makes up around 23% of the mass-energy content, so the discrepancy is due to the existence of non-baryonic dark matter. This nicely fits in with our observations of galaxies and clusters that show that most of the matter (approximately 5/6th) is in a form we can not see.
So what might dark matter be made of? There are many different hypothetical particles proposed to explain dark matter, but the leading candidates are known as Weakly Interacting Massive Particles, or WIMPs. WIMPs interact with baryonic matter through gravity (as we know dark matter does), and are also expected to interact very slightly through a force known as the weak nuclear force. Simulations predict that dark matter made of WIMPs would produce structures in the Universe that are very similar to what we actually observe. If WIMPs really do interact through the nuclear force, scientists may be able to detect them directly using sensitive underground detectors. There must be many dark matter particles passing through the Earth all the time, and although most pass unimpeded occasionally one may interact with a molecule, producing a tiny flash of light and new decay particles. By looking for these telltale flashes and working out the identity of the decay particles produced in the reaction, it may be possible to deduce the identity of dark matter itself.
Author: Emma Grocutt