JOIN USWith the discovery that the expansion rate of the universe is accelerating, it has become evident that most of the universe is constituted of material which exerts a negative pressure that makes gravity repulsive. This could just be Einstein’s cosmological constant, observes C Sivaram
Recent observations continue to support the current cosmological picture of a universe driven by dark energy (DE). This makes the universe expand even faster as time passes, i.e. it is accelerating. The 2011 Nobel Prize in Physics was awarded to three astronomers for establishing, after more than a decade of observation, that the expansion of the universe is actually faster now than it was several billion years ago. They based this on the observation of distant supernovae (i.e. exploding stellar objects releasing titanic energies) which appeared fainter than they should, suggesting that the distance that separates us from them is increasing at an accelerating rate.
The normal tendency of gravity is to pull together objects which are moving apart. This is also borne out by the simple example of a stone thrown upwards falling back after a time, so astronomers expected the expansion rate to slow down, i.e. to decelerate. The expansion rate should have been faster several billion years ago tending to slow down and ultimately halt in the future. So what could make the universe expand faster and faster? One possibility was, in fact, suggested by Einstein several years before Hubble’s discovery of an expanding universe. He believed in a static unchanging universe and found that his equations of general relativity would not produce such a model unless he introduced a large scale repulsive force to counteract the attractive gravity pulling all the masses together.
He called it the cosmological constant which essentially introduced a negative pressure which makes gravity repulsive. After the universal expansion of the cosmos was established, Einstein realised that he would have predicted such a dynamic universe if he had not introduced his cosmic repulsive term and he is even said to have called it his biggest blunder!
Then Eddington, Lemaitre and others realised that one could still have an expanding universe even while retaining the cosmological constant which would then be an additional parameter (to be determined by observations). Hubble’s earlier estimate of the expansion gave too low an age for the universe and it was understood that the cosmic term could resolve this.
Most matter is dark
With the discovery over just a decade ago that the expansion rate of the universe is increasing, i.e. accelerating, it became evident that most of the universe is constituted of material which exerts a negative pressure making gravity repulsive. This is dark energy, which makes up at least 70 per cent of the matter in the universe. Ironically very recent observations (for instance by Max Planck Institute and University of Portsmouth) support the fact that this DE may just be Einstein’s cosmological constant. The expansion rate is consistent with this.
Although Einstein did not want to retain his cosmic repulsion, subsequently, it was realised that quantum vacuum energy (in quantum theory, a vacuum is not just absence of particles but production of more and more particles as space-time is squeezed to smaller length scales) has exactly the same mathematical form as Einstein’s extra term.
Quantum vacuum exerts a negative pressure. In an expanding universe, especially at early times, this would have been dominant (causing the expansion in the first place).
The first evidence for DM was provided by astronomer Fritz Zwicky, a few years after Hubble’s discovery of the expansion of the universe. He noted that large clusters of galaxies seemed to contain vast amounts of matter not contributing to the stellar radiation. In some cases, it was even about a 100 times more than the luminous matter.
Such amounts of matter kept the galaxies in the cluster moving about at well above a 1,000 km per second. Several decades later, it was observed that even individual galaxies, like our Milky Way, require about ten times as much DM as visible matter (in stars etc) to keep objects far from the galaxy centre orbiting at undiminished velocities of well above two hundred kilometres per second.
Still a mystery
What is DM is still a mystery. A new stable particle, several times the proton mass, produced in the beginning of the universe may be involved. The Large Hadron Collider searches for these particles as the energies correspond to the early universe.
How will such a dark universe evolve? If, as current observations strongly suggest that the DE is nothing but a constant cosmological constant, the universe would soon start expanding faster at an exponential rate, so that after 20 billion years, only the local group of galaxies would be visible to us, all the rest of the universe would be beyond the range of even the biggest possible telescope.
In a trillion years or so, even all the stars in our galaxy (including the long lived low mass stars) would have ceased to shine. No more new stars would form. If an astronomer still exists somewhere, he would see a dark sky with stygian darkness. This is the Big Freeze scenario. However if DE grows in time, its increasing negative pressure would rip everything apart including atoms or nuclei, the so-called Big Rip scenario.
Early findings
As early as 1975, this author, with his senior colleagues (Sinha and Sudarshan), proposed a domination of the quantum vacuum energy which prevails in the universe at present, accounting for most of the so-called dark matter (DM). This work was published in international journals. Quantum vacuum energy (equivalent to Einstein’s cosmic term) dominates the universe and could account for the DM. The calculated magnitude of the DE agrees with the present observed estimates) after DE became an acceptable paradigm!