How the universe will end

By Kumar P Mainali

There is a bad news and a good news for the universe dwellers. The bad news is that nothing lasts forever, and so the universe. We are not sure whether the universe will disappear but it is quite certain that over the time, it will get increasingly discomfortable for life and ultimately unlivable.

And the good news is that this will not happen till next 100 billion years. The universe is likely to remain hospitable to life for this period which is 20 times as long as the earth has existed.

Milky Way is a huge, whirling pinwheel made of 100 billion or more stars, and tens of billions of other galaxies lie beyond its edges. The most astonishing fact about them is that these galaxies are rushing away from one another right after the explosive cataclysm known as the Big Bang. Will the galaxies continue to fly apart forever, their glow fading until the cosmos is cold and dark? Or will the expansion slow to a halt, reverse in direction and crash all the 10 octillion (10 trillion billion) stars in a final Big Crunch? Scientists haven’t been able to decide.

The classic Big Bang theory (now some scientists prefer to call it Big Bang model), refined over the decades since the astronomer Edwin Hubble discovered the expanding universe in 1929, suggests that cosmic destiny will be decided through a tug-of-war between two opposing forces. One is the expansion of space, which has been carrying galaxies ever farther apart from one another. The other is the mutual gravitational force exerted by those galaxies and all the other stuff in the universe which acts as a brake, slowing down the expansion rate.

As the universe expands, the combined gravity from all the matter within it tends to slow the expansion, much as the earth’s gravity tries to pull a rising rocket back to the ground. If the pull is strong enough, the expansion will stop and reverse itself; if not, the cosmos will go on getting bigger forever. The former ultimately dissolves the universe into a fireball. This Big Crunch would melt everything, even subatomic particles. The latter makes the universe unpleasantly dark and cold. Over a long period of time, the hydrogen and helium in stars will run low and old stars without enough light are not replaced by new ones. The universe gradually fades to black and cold.

Whether the expansion or gravity will win in the end is not clear. Most observations favour the former, but many uncertainties persist. The most important of them is the ‘dark matter’ issue.

In 1930s it was noted that something beside glowing stars and gases is present in the universe. This was found out by studying the movement of galaxies. The galaxies in clusters were orbiting one another too fast than expected which could not be explained on the basis of gravitational force produced by the visible matter. Individual galaxies were also spinning about their centers too quickly. The greater the gravitational force of attraction is, the faster the galaxies move. Thus the visible matter would not be able to hold the observed orbiting movement of galaxies. Instead the galaxies should long since have flown apart. The only possible reason to explain this is extra gravitational force produced by invisible dark matter.

Study suggests that the visible matter (stars and nebulae) constitute only 1% to 10% of the total matter in the universe. The rest is invisible. It is invisible because it emits no light. Although studies are able to explain up to this point, nobody yet knows what this dark matter is composed of. One possibility is that it’s made of weakly interacting massive particles (WIMP). But until the dark matter is identified, predicting the future of the universe on the basis of what we can see will be highly erroneous. As the extragravity produced by dark matter would explain the faster orbiting and spinning of galaxies, it together with the gravity produced by visible matter should have also slowed down the expansion of the universe over the time.

So astrophysicists tried to determine whether the expansion was slowing down and by how much. In 1995, two groups of astronomers, the Supernova Cosmology Project and the High-z-Supernova Team, set out to find it. They wanted to measure the cosmic slowdown, known formally as "deceleration parameter".

But in 1998, both of the groups individually came out with shocking results: the universe is expanding faster than earlier (accelerating) in spite of the inevitable slowing down induced by the gravity’s pull. If it is correct, in billions of years many of the stars will be gone from the night sky and the universe will be a very lonely place to look at. It will be very dark and cold.

The common wisdom before 1998 was as follows: The light we see from distant objects was emitted a long time ago, when the expansion was faster, and therefore the measured speeds of such objects should be higher than the speeds of nearby objects. The idea of both of the research teams was straightforward: look at the nearby universe and measure how fast it is expanding. Then do the same for the distant universe, whose light is just now reaching us, having been emitted when the cosmos was young. Then compare the two.

The principle underlying the observational strategy is simple. Towards the end of life when the fuel (hydrogen and helium) in the star finishes, star like sun turns into a very small dense body emitting very little light. These stars are called white dwarfs. A white dwarf may remain so for 100 trillion years - or a thousand times longer than the cosmos has existed to date, if it is in isolation. But if it has a companion star, this companion star can transfer matter to the white dwarf. Once the white dwarf reaches the mass of 1.4 solar masses, nuclear burning of carbon (which is the main constituent of the dying star) starts. This results in explosive dispersion of the white dwarf’s contents into the interstellar medium, from which later generations of stars are formed.

One may ask whether the supernovae may somehow fool us into believing that we are observing accelerated expansion. The observed effect is simply that the more distant supernovae are dimmer than expected by about 25%; could this dimming be caused by other effects than accelerated expansion? This is a very relevant question. But so far, there are no suspicions.

These two teams of astronomers studied 14 supernovae, 7 billion to 10 billion light years from Earth. More observations of distant supernovas and more theoretical work will be required before one can safely conclude that the cosmic expansion is indeed accelerating and the fate of the universe is ice. Once the Next Generation Space Telescope (NGST) is launched in few years, detection and identification of supernovas at even greater distances and higher redshifts will become possible. Then I will tell you how the universe will end : in fire or in ice.

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