# Fate of the Universe

 Figure 13. The rate of expansion of the universe since its birth 15 billion years ago. The shallower the curvature, the faster the rate of expansion. The curvature changes noticeably about 7 billion years ago, when objects in the universe began flying apart at a faster pace. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart. Image Credit: NASA/STSci/Ann Feild.

In his book The Emperor’s New Mind, mathematician and cosmologist, Roger Penrose, described how cosmology matured from a ‘speculative pursuit’ into an ‘exact science’ following the discovery of CMB. Today, with the advent of satellite technology and high resolution telescopes, scientists are designing extraordinarily delicate experiments that have the capability of probing deep into space by looking at ancient stars and galaxies that were formed few thousands of years after the Big Bang. The BICEP2 results are currently controversial and may require several years to build up a sizable statistical significance. But if confirmed, they could conclusively prove the role of inflation in the early moments of Big Bang when the universe went through a period of exponential expansion. Such an inflationary phase provides an explanation of why the universe looks the same in every direction, because, in the early universe, there would be enough time for light to travel from one region to another.

Edwin Hubble showed that the universe is expanding as a consequence of the Big Bang. Saul Perlmutter, Brian Schmidt, and Adam Riess and their teams studied over 50 distant supernovae whose light was weaker than expected, and concluded that the expansion of the universe is accelerating, which is truly an astounding result. Moreover, the universe is expanding faster now than it was 7 billion years ago (see Figure 13 where the curvature of expansion changes noticeably around about 7 billion years ago). The acceleration is thought to be driven by dark energy, but not much is known about dark energy.

 This short clip shows the evolution of the universe after Big Bang. Notice how the expansion of the universe slows down to a halt and then starts expanding at a faster and faster pace mimicking the effects of gravitation and dark energy. Video Credit: Chandra X-ray Center, Operated for NASA by the Smithsonian Astrophysical Observatory.

Scientists now believe that the universe expanded extremely fast after the Big Bang when it was much smaller and denser than it is today and since matter in the universe was so close together, gravity began slowing the universe’s expansion. The mysterious dark energy always existed in the early universe but was overwhelmed by a stronger presence of gravity. It became significant when the universe expanded to a point when the distance between masses became greater causing the effect of gravity to weaken and the power of dark energy took over causing the expansion to speed up.

Dark energy is perhaps the greatest mystery in physics today as it constitutes about three quarters of the universe.

The WMAP and Planck satellites measured the cosmological parameter $\sigma_0$ to be $1.02 \pm 0.02$ indicating that the geometry of the universe is essentially flat, implying that the data strongly favor an ever expanding universe.

Can we tell anything about the fate of our universe billions of years from now?

Lawrence Krauss and Robert J. Scherrer have tried to predict the position of the galaxies in the cosmos 100 billion years into the future. As the universe expands, more space will be created as the progressively stronger dark force pushes the galaxies further apart at a faster and faster rate. Krauss and Scherrer speculate galaxies will completely disappear from view as they approach the speed of light, as if they have fallen into a black hole after crossing its event-horizon. Light from the galaxies will be massively red-shifted with infinitely long wavelengths and dimmed by their great speeds. The most distant galaxies will disappear first except for the few galaxies that are bound together gravitationally into the Local Group 16. Those will be the only ones visible in the sky.

In 1987, George Ellis and Tony Rothman showed even ordinary expansion would gradually push most galaxies too far away to be detected, setting the stage for cosmic ignorance. Dark energy only hastens the process. The universe might be heading toward a state that is “asymptotically empty”.

The CMB radiation will be red-shifted (peak wavelengths of the CMB will be on the scale of meters, corresponding to radio waves instead of microwaves) and blurred by the radio noise of our own galaxy. The deuterium (a heavy form of hydrogen) that was once manufactured during the Big Bang and currently abundant in deep space will now become totally invisible because it will not be backlit by the distant quasars; both deuterium and quasars will have passed beyond the event horizon and will be lost forever to view.

Interestingly, before the universe was 7 billion years old, dark energy was unobservable and 100 billion years from now it will become unobservable again, but for a totally different reason. In the first case, there was no accelerating expansion of the universe, and in the latter, the universe will become too large for light to have time to reach an observer at his or her vantage point.

Imagine what future cosmologists will see (provided the human race manages to escape from the solar system before the death of the sun in 5 billion years) from their vantage point in the night sky billions of years from now! The Local Group galaxies will have merged to form one large galaxy, and essentially all other galaxies will be long gone, having escaped beyond the event horizon. Future cosmologists will see the stars of our galaxy but the largest and brightest stars that we see today will have exhausted their nuclear fuel, although smaller stars will still light up the night sky.

 The quickening expansion will eventually pull galaxies apart faster than light, causing them to drop out of view. This process eliminates reference points for measuring expansion and dilutes the distinctive products of the Big Bang to nothingness. In short, it erases all the signs that a Big Bang ever occurred. To our distant descendants, the universe will look like a small puddle of stars in an endless, changeless void. — The Editors, Scientific American

Armed with knowledge accumulated in eons, the future cosmologists will certainly possess a knowledge base much superior to ours but if general relativity is correct, they would not be able to build telescopes to view galaxies at the edge of the universe! In 100 billion years, Hubble’s crucial discovery of the expanding universe will become irreproducible. And so will be the strange phenomenon of accelerating expansion because the reference points for measuring this expansion will be eliminated forever.

The loss of information is unavoidable in the distant future because of the cosmic acceleration (this will occur regardless of whether the rate of accelerated expansion varies or not as the universe ages) unless future advances in science and technology can somehow open up a window in the universe’s past. One can only speculate how a civilization living in an unimaginably distant future would eventually develop such advanced technologies and what those technologies might even be.

16 The Local Group is the group of galaxies that includes the Milky Way among others. It comprises more than 54 galaxies, counting dwarf galaxies. Its gravitational center is located somewhere between Milky Way and Andromeda Galaxy covering a diameter of 106 light-years.