The Milky Way, and (below) Krauss (right) and Scherrer have been greatly acclaimed for their prediction of a ‘static universe’

It’s an occurrence that shouldn’t be bothering us. Long after humankind is dead and gone and the sun — the sole source of energy in the solar system — burnt out, the universe will not appear to be the same as it does today. The visible universe is destined to shrink to an abysmally small size, say researchers. Besides, it would appear static too.

The Milky Way galaxy and its immediate galactic neighbourhood are bound to get lost in the huge expanse of dark void in a few trillion years, says Lawrence Krauss, a cosmologist at Case Western Reserve University in Ohio, the US. That is because this local group of galaxies, that stays together due to gravitational pull, would have moved so far from the rest of the universe — and that too at a faster rate than the speed of light — that no information (such as light) will reach it. The disappearance of the information that currently allows us to see how the universe expands over the visible horizon may thus give the false notion of a “static universe” — a perception commonly held by astronomers at the turn of the 20th century.

What remains will be “an island universe” consisting of the Milky Way, Andromeda (our nearest galaxy) and a few puny galaxies that will form a galactic Local Group, says Robert Scherrer, theoretical physicist at Vanderbilt University, and a co-author of the study.

To track motion, we need markers. Consider a person in a glass enclosure. The person cannot feel the wind outside. The only way he can perceive the wind blowing is by seeing the motion of objects that are being blown around by it. With the Local Group being so far away, no such markers will be available to observers from galaxies in the group, says T.R. Seshadri, an astrophysicist at the University of Delhi.

Incidentally, our solar system would be long gone before this happens. The sun is almost close to half its life, which is estimated to be 10 billion years. Krauss and Scherrer let their imagination run wild by presuming that there will be earth-like habitable planets around other stars at that point in time, with scientists studying the universe with instruments similar to those available today! (Both the Milky Way and Andromeda consist of billions of stars and some of them may be sun-like.)

In such a scenario, many observational pillars on which modern cosmology are propped up — such as the cosmic microwave background radiation from the afterglow of early universe formation and the movement of galaxies away from the Local Group — will not be available to them, they argue. “While physicists of the future will be able to infer that their island universe has not been eternal, it is unlikely that they will be able to infer that the beginning involved a Big Bang,” says Krauss.

This prediction of a “static universe” has won Krauss and Scherrer the annual prize of the Gravity Research Foundation, whose earlier recipients included Stephen Hawking and Roger Penrose.

Subhabrata Majumdar of Mumbai’s Tata Institute of Fundamental Research, however, does not think this is a “significant piece of work”. But, of course, it has popular appeal, he says.

Ironically, the reason for the perceived “static” nature of the universe is related to the ever-increasing speed at which the universe is expanding. According to the current theoretical models of cosmology — which are built on Einstein’s General Theory of Relativity — the initial momentum for this unhindered expansion came from a fiery Big Bang explosion that occurred some 13.7 billion years ago. However, ever since, the acceleration has picked up, thanks to a mysterious repulsive field or energy — called dark energy — that scientists think permeates space. Dark energy, which caught the attention of scientists about a decade ago, can be described as the growing tendency of empty space to create more empty space, thereby distancing anything in the universe that is not bound by gravity. Researchers estimate that 70 per cent of the universe is made up of dark energy.

“If the universe expands in an accelerated way, the number density of galaxies will ultimately become so small that we will not see the distant ones,” Seshadri told KnowHow. According to Krauss and Scherrer, this would begin to occur over the next 100 billion years or so.

Similarly, around the same cosmological time scale, detecting the cosmic background radiation that convinced most physicists and astronomers that there was, in fact, a hot Big Bang would be next to impossible. “The intensity of radiation (around 100 billion years later) would be smaller by a million million times, which means detectors will have to be a million million times more sensitive,” says Krauss. Then, later, the wavelength will be even longer and since such radiation is absorbed in the galaxy, it cannot make its way to us.

This would also mean the death knell for modern cosmology. “Cosmology is an empirical science, just like the rest of physics. That is what makes it so remarkable, namely, that we can actually observationally probe the universe and learn about the past and the future. When these observables disappear in the distant future, cosmology as an empirical science will be largely over,” Krauss told KnowHow.

Krauss, a noted popular science writer and an ardent champion of science who once wrote to the Pope appealing him to “not build walls” between science and religion, thinks that we’re living in a special time in the evolution of the universe. If the universe was an order of magnitude younger, observers would not have been able to discern any effect of dark energy on the expansion. Similarly, when the universe is more than an order of magnitude older, observers will be hard pressed to know that they live in an expanding universe, as dark energy would have been all pervading.