Topics: Dark Matter, Dark Energy

Dark Matter, Dark Energy

Dark Energy, Dark Matter




What Is Dark Energy?





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.

What Is Dark Matter?






When the Universe was young, it was nearly smooth and featureless. As it grew older and developed, it became organized. We know that our solar system is organized into planets (including the Earth!) orbiting around the Sun. On a scale much larger than the solar system (about 100 million times larger!), stars collect themselves into galaxies. Our Sun is an average star in an average galaxy called the Milky Way. The Milky Way contains about 100 billion stars. Yes, that's 100,000,000,000 stars! On still larger scales, individual galaxies are concentrated into groups, or what astronomers call clusters of galaxies.

The cluster includes the galaxies and any material which is in the space between the galaxies. The force, or glue, that holds the cluster together is gravity -- the mutual attraction of everything in the Universe for everything else. The space between galaxies in clusters is filled with a hot gas. In fact, the gas is so hot (tens of millions of degrees!) that it shines in X-rays instead of visible light. In the image above, the hot X-ray gas (shown in pink) lying between the galaxies is superimposed on an an optical picture of the cluster of galaxies. By studying the distribution and temperature of the hot gas we can measure how much it is being squeezed by the force of gravity from all the material in the cluster. This allows scientists to determine how much total material (matter) there is in that part of space.

Remarkably, it turns out there is five times more material in clusters of galaxies than we would expect from the galaxies and hot gas we can see. Most of the stuff in clusters of galaxies is invisible and, since these are the largest structures in the Universe held together by gravity, scientists then conclude that most of the matter in the entire Universe is invisible. This invisible stuff is called 'dark matter'. There is currently much ongoing research by scientists attempting to discover exactly what this dark matter is, how much there is, and what effect it may have on the future of the Universe as a whole.

It's Dark Out There....

"Dark matter" refers matter of an unknown type that astronomers and cosmologists believe must make up the majority of the mass in the universe. It is called "dark" because it does not emit any light. We know of its presence because of the gravitational effects it has on objects that we can see. For example, galaxies in clusters move at speeds that are too high to be attributed just to the visible galaxies. In addition, astronomers measure high temperature gas in these galaxy clusters. This gas is at too high a temperature to remain bound to the cluster without some additional, hidden, mass. For galaxies and groups, the X-ray data have often indicated very extended dark matter halos far beyond the radius at which one sees starlight or galaxies. The total inferred dark matter mass is often 10 times that in the "visible" galaxies alone.

Dark matter also plays a role in the early universe. Astronomers theorize that the presence of dark matter helps to explain the relative amounts of light elements and isotopes produced in the Big Bang. Results from the Wilkinson Microwave Anisotropy Probe (WMAP) show that 23% of the Universe is made up of dark matter.

One of the best ways of determining the mass of a system, such as a cluster of galaxies, group of galaxies or a massive elliptical galaxy, is to measure the X-ray temperature and gas profiles. Astronomers start by assuming the gas is in equilibrium, which is borne out by the thermal spectra of the gas. Matching models of the distribution and temperature of the gas to the X-ray observations gives the mass of the gas. Use of this technique has shown that clusters of galaxies are gas and baryon rich, that is, the mass in gas exceeds the mass in stars by factors of 3-5 and that the total baryonic mass is ~15% of the total mass of the cluster (i.e. visible mass plus dark matter). Since clusters are supposed to be "fair samples of the universe" they should have a baryon fraction that corresponds to 4%, as inferred from Big Bang nucleosynthesis and results from WMAP. However, WMAP and other evidence now point to a new component in the universe, which is called dark energy. Dark energy makes up 73% of the energy and matter of the universe. By including this dark energy with the visible mass and dark matter, we find that clusters really do share in the same baryonic fraction as the rest of the universe.



Dark matter Not dark matter





* Baryonic

* Non-Baryonic

o hot dark matter (HDM) and

o cold dark matter (CDM),


Aside on flatness





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 achieve 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 universe. 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 Matter, Dark Energy
Most of the universe seems to consist of nothing we can see. Dark energy and dark matter, detectable only because of their effect on the visible matter around them, make up most of the universe. Source: