The long-term future of extragalactic astronomy could be dark and dreary, according to new theoretical research. Such a prediction might have not have been taken so seriously, had it not originated from Abraham Loeb of the prestigious Harvard-Smithsonian Center for Astrophysics. Professor Loeb’s assertion is based on a serious study of the evolution of how we will see the universe in the future.
According to the current cosmological model, some 70% of the universe could be composed of “dark energy” acting as a repulsive force opposing gravitation. If this were true, then rather than slowing down its expansion, the universe would actually be entering a phase of accelerated expansion similar to the inflation that lasted for a split second soon after the Big Bang. Consequently, over the next few billion years, the remotest galaxies would pass beyond our cosmic horizon. They would not suddenly disappear over the horizon, but their image would become frozen while gradually fading away. “This process is analogous to what you see if you watch a light source fall into a black hole,” explains Loeb. “As an object crosses the black hole’s event horizon, its image seems to freeze and fade away because you can’t see the light it emits after that point.”
In some distant future, this will have ominous consequences for our study of the universe. Not only will the number of observable galaxies decrease, but we will also be unable to observe their evolution beyond a certain time. Take the example of the most distant quasar observed to date. The light we receive from it left the quasar when the universe was only about a billion years old. If we observe this quasar over the next few billion years, we will see it solidifying and disappearing slowly at an age of only 6 billion years, which is less than half the universe’s current estimated age of 14 billion years. So, even by observing distant sources over extremely long time scales, we will always have to rely on indirect arguments to connect the properties of such objects to those of nearby galaxies. Similarly, a light signal sent today from Earth will never be able to reach a galaxy currently observed with a spectral shift higher than z = 2.
In less than about 100 billion years, the only galaxies still visible will be those gravitationally linked to the local group of galaxies, including the Virgo cluster and perhaps part of the local supercluster. All other light sources will have frozen in time and faded to invisibility.
