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Tools for Unlocking the Secrets of the Universe

May 18, 2004 ::

Abell 2029

Recent Chandra X-ray Observatory results provide compelling new evidence for an accelerating universe. They give astronomers yet another tool, along with optical observations of distant supernovas, microwave studies, and galaxy surveys, to probe the expansion of the universe and the nature of dark energy.

Energy Distribution of the Universe

Cosmological observations over the last 75 years indicate that we live in a universe that began in a very hot dense soup known as the Big Bang. The universe has been expanding since the Big Bang for the last 13.7 billion years. The geometry of the universe is flat, which means that no matter where you are in the universe, the sum of the angles of a triangle of any size will be 180 degrees. The contents of the universe are ordinary matter such as protons and neutrons, dark matter which is observable only through its gravitational effects, and dark energy. Dark energy is the strangest of the 3 constituents, and comprises about 75% of all the energy in the universe.

It is worthwhile to take a closer look at how the effects of dark energy were discovered. [For more information on dark energy and acceleration, please see the interview with Professor Michael Turner from the University of Chicago.]

Although there were tantalizing hints that some form of dark energy might exist, it wasn't until 1998 that the first strong evidence was found. In 1998 two teams of scientists studying a particular class of supernovas published their amazing results.

Accreting White Dwarf

When a small ancient star known as a white dwarf has a companion star in close orbit around it, matter can be pulled from the companion star onto the white dwarf. If the infall occurs rapidly enough, the white dwarf will reach a critical mass limit of 1.4 solar masses (the Chandrasekhar limit - discovered by Chandra's namesake!) and subsequently explode. Because the mass of the exploding star for this class of supernova, called Type Ia, is always the same, the intrinsic brightness of the resulting explosion is also the same.

A Type Ia supernova is called a "standard candle" for this convenient trait, and can be used to determine the distance to supernovas. For example, if one such supernova appears to be dimmer than another, it must be further away by an amount that can be computed from the difference in apparent brightness of the two supernovas.

The scientists measured the brightness of many distant supernovas and found them to be significantly dimmer than expected. This meant that the explosions occurred significantly further away. The working assumption of the astronomical community up until this time was that the expansion of the universe was slowing down, due to the gravitational pull of the matter. The question at the time was "how much?" The supernova teams found that the universe behaved in the exact opposite way - speeding up, pushing distant objects even further away. Thus, the first shocking piece of evidence fell into place.

The Microwave Sky image from the WMAP Mission

In the following years, other results pointed toward an accelerating universe, most notably the Wilkinson Microwave Anisotropy Probe (WMAP), which studied small fluctuations in the cosmic microwave background radiation (CMB). The CMB is radiation that is left over from the hot, dense early universe. At the earliest times, particles and photons bounced around in the dense soup but as the universe expanded it cooled until eventually protons and electrons could combine to form neutral hydrogen. At this point the radiation (photons) did not have particles to interact with, so it became decoupled from matter, spreading freely throughout the universe. This radiation is now very cold, only 2.7 degrees above absolute zero. But it has much information hidden inside. By measuring the tiny fluctuations in the temperature of this bath of radiation scientists determined that the universe must be made up of about 4% ordinary matter and 21% dark matter. A full 75% of the energy in the universe was not accounted for, and this fact, along with corroborating evidence, pointed toward a universe filled with a mysterious dark energy.

Galaxy Cluster Animation

The final piece of evidence, making the conclusion for an accelerating universe inescapable, is an independent method in a completely different energy band: X-ray observations of the largest objects in the universe, huge clusters of galaxies. This method was first proposed in 1996 but could only be undertaken with the launch of Chandra in 1999. Large galaxy clusters are made up of both ordinary and dark matter, with much of the ordinary matter in the form of hot intergalactic gas (in fact 6 times more gas than stars and galaxies!) which glows in X-rays. Because clusters are so large, the amount of ordinary matter compared to dark matter in these clusters should be constant whether the cluster is close to us or very far away. Computer simulations and a feasibility study comparing 15 nearby galaxy clusters with 15 distant clusters gave strong support to this idea.

Fortunately for astronomers, calculating the amount of ordinary matter in a galaxy cluster requires knowing the distance to the cluster. Whenever large distances are involved, the existence of dark energy will have an effect on the result. Thus, knowing what the fraction of ordinary matter to dark matter in the clusters should be, scientists could calculate this fraction using different distance scales. Since each distance scale corresponds to a different type of universe, the correct mass fraction will result only if the correct type of universe is assumed. The Chandra observers did this calculation for 26 of the largest clusters of galaxies. Their results clearly showed that the universe we live in is expanding at an accelerating pace.

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