JOIN USIf Dark Matter makes up one-fourth of the matter in the Universe, what about the remaining three quarters? C Sivaram explains...
Dark matter (DM) is stated to constitute one-fourth of the matter in the Universe. So far, there is no unambiguous evidence as to what it is made up of or what type of particles constitute it.
This is despite the fact that there are several ingenious experiments currently running in many parts of the world finding answers for the question: If DM makes up one-fourth of the matter in the universe, what about the remaining three quarters?
The type of matter familiar to us in our everyday life and of which we are made up of – atoms and molecules makes up less than five per cent of the matter in the Universe. This is called baryonic matter. Baryons are either protons or neutrons which collectively make up the nuclei of the atoms of the ninety two naturally occurring elements in the periodic table. The nuclei have orbiting electrons that make the atoms neutral.
The baryonic matter is about three-quarters hydrogen, about one fourth of helium and the remaining one or two per cent constitutes all the other heavier elements.
Heavy elements like gold or uranium constitute only a small fraction of baryonic matter. For example, there is only one gold atom for ten billion hydrogen atoms, in the Universe. Stars like the Sun, planets and all biological entities are made of baryonic matter; we ourselves are three-fourths water.
But baryonic matter, as a whole, makes up hardly five per cent of the universe. For a moment, go back to the Big Bang event of the Universe where it began with a very hot dense phase, the temperature being about ten billion degrees, one second after it started expanding. This model predicts that the temperature of the radiation drops as the Universe continually expands.
In other words, we should be surrounded by a ubiquitous comic microwave background radiation (CMBR). The discovery of such a background in 1965, with a temperature of about three degrees Kelvin, lent considerable support to this theory. Subsequent studies have confirmed the theory and brought in two Nobel Prizes making it a mainstream field of research.
Subsequently, it was shown that under the super hot conditions in the early Universe, about one-fourth of the hydrogen would have been converted into helium by thermonuclear reactions. Trace amounts of deuterium (heavy hydrogen) and a lighter isotope of helium would also have been synthesised by the primordial nuclear reactions.
Now, in stars, only about two per cent of hydrogen would get converted into helium, so if stars are the only source for helium, all the stars put together can hardly convert two per cent of hydrogen to helium.
Proving the Big Bang theory
However, even the oldest stars, show much higher helium abundance, suggesting that most of the helium is primordial. Again, deuterium and lithium are not made in stars, but are only intermediate products in the complex reactions. But even the oldest stars and interstellar gas contain deuterium and lithium in trace amounts which agrees closely to what is produced in the big bang.
However, all this works only if the baryons make up only four per cent of the matter. Peaks in the microwave background provide strong evidence that the Universe is close to what is called the critical density.
Evidence from clusters of galaxies and individual galaxies suggest that the Universe is about one-fourth dark matter. This, together with the baryonic matter makes up only about thirty per cent of the matter required to make up the critical density. So, what about the remaining seventy per cent? This is supposed to be the so-called dark energy (DE).
The evidence for such an exotic form of matter, that is, dark energy, comes from the discovery (over the past fifteen years) that the expansion of the Universe is actually accelerating. The Universe is expanding faster now than it was, say eight billion years ago. This was based on observations of very distant supernovae.
These type of supernovae release a very definitive amount of energy, and how bright they appear indicates their distance from earth. In an accelerating universe, the supernovae appear fainter than expected as distance of separation is increasing faster with time. Dark energy (DE) differs from dark matter (DM) in that it gives rise to negative pressure, which causes gravity to become repulsive and causes particles to accelerate faster from each other.
From the difference in the rates of expansion eight billion years ago, when the supernovae exploded and at present, it was concluded that seventy per cent of the Universe is dark energy. It is this seventy per cent domination by dark energy that makes the Universe an accelerating one, expanding faster.
This was a surprise discovery (Science magazine called it the number one discovery some years back) and resulted in the 2011 Nobel Prize to three scientists connected with it.
Dark matter and baryonic matter are too little to make the Universe to decelerate and collapse. So, what is this dark energy? It was Einstein who introduced such a repulsive force (negative pressure) while considering a static Universe in his
theory.
Revival of the concept
As gravity is universally attractive, a static Universe would only result if there is a corresponding repulsion on a large scale to balance it. So, Einstein introduced the cosmological constant in his equations. After Hubble’s discovery of an expanding Universe, this repulsive cosmic term was abandoned by most cosmologists.
But, now it appears that much of the current observations is consistent with such a repulsive cosmological constant dominating the Universe to the extent of seventy per cent or more. This may well be the dark energy.
There are other suggestions like a varying scalar field, quintessence – the fifth state of matter, exotic fluids (Chaplygin gas) and so on.
But as in the case of DM, we are still in the dark! It is baffling to note that we are still not sure of what constitutes ninety-five per cent of the matter in the Universe (DM and DE together).
Many have suggested that Einstein’s general theory of relativity may itself have to be modified. But one thing is for sure. Many exciting future discoveries await us in cosmology.