Habitable Universes

We know that several of the distinctive features of the large scale structure of the visible universe play a role in meeting the conditions needed for the evolution of biochemical complexity within it.

The first example is the proximity of the expansion dynamics to the ‘critical’ state which separates an ever-expanding future from one of eventual contraction, to better than ten per cent. Universes that expanded far faster than this would be unable to form galaxies and stars and hence the building blocks of biochemistry would be absent. The rapid expansion would prevent islands of material separating out from the global expansion and becoming bound by their own self-gravitation. By contrast, if the expansion rate were far below that characterising the critical rate then the material in the universe would have condensed into dense structures and black holes long before stars could form.

The second example is that of the uniformity of the universe. The non-uniformity level on the largest scales is very small, Δ ≈ 10^-5. This is a measure of the average relative fluctuations in the gravitational potential on all scales. If Δ were significantly larger then galaxies would have rapidly degenerated into dense structures within which planetary orbits would be disrupted by tidal forces and black holes would form rapidly before life-supporting environments could be established. If Δ were significantly smaller then the non-uniformities in the density would be gravitationally too feeble to collapse into galaxies and no stars would form. Again, the universe would be bereft of the biochemical building blocks of life.

In recent years the most popular theory of the very early evolution of the universe has provided a possible explanation as to why the universe expands so close to the critical life-supporting divide and why the fluctuation level has the value observed. This theory is called ‘inflation’. It proposes that during a short interval of time when the temperature was very high (say 10²⁵K), the expansion of the universe accelerated. This requires the material content of the universe to be temporarily dominated by forms of matter which effectively antigravitate for that period of time. This requires their density r and pressure, p, to satisfy the inequality

(4)

The inflation is envisaged to end because the matter fields responsible decay into other forms of matter, like radiation, which do not satisfy this inequality. After this occurs the expansion resumes the state of decelerating expansion that it possessed before its inflationary episode began.

If inflation occurs it offers the possibility that the whole of the visible part of the universe (roughly 15 billion light years in extent today) has expanded from a region that was small enough to be causally linked by light signals at the very high temperatures and early times when inflation occurred. If inflation does not occur then the visible universe would have expanded from a region that is far larger than the distance that light can circumnavigate at these early times and so its smoothness today is a mystery. If inflation occurs it will transform the irreducible quantum statistical fluctuations in space into distinctive patterns of fluctuations in the microwave background radiation which future satellite observations will be able to detect if they were of an intensity sufficient to have produced the observed galaxies and clusters by the process of gravitational instability.

As the inflationary universe scenario has been explored in greater depth it has been found to possess a number of unexpected properties which, if they are realized, would considerably increase the complexity of the global cosmological problem and create new perspectives on the existence of life in the universe.

It is possible for inflation to occur in different ways in different places in the early universe. The effect is rather like the random expansion of a foam of bubbles. Some inflate considerably while others hardly inflate at all. This is termed ‘chaotic inflation’. Of course, we have to find ourselves in one of the regions that underwent sufficient inflation so that the expansion lasted for longer than t_* and stars could produce biological elements. In such a scenario the global structure of the Universe is predicted to be highly inhomogeneous. Our observations of the microwave background temperature structure will only be able to tell us whether the region which expanded to encompass out visible part of the universe underwent inflation in its past. An important aspect of this theory is that for the first time it has provided us wit ha positive reason to expect that the observable universe is not typical of the structure of the universe beyond our visible horizon, 15 billion light years away.

It was subsequently been discovered that under fairly general conditions inflation can be self-reproducing. That is, quantum fluctuations within each inflating bubble will necessarily create conditions for further inflation of microscopic regions to occur. This process or ‘eternal inflation’ appears to have no end and may not have had a beginning. Thus life will be possible only in bubbles with properties which allow self-organized complexity to evolve and persist.

It has been found that there is further scope for random variations in these chaotic and eternal inflationary scenarios. In the standard picture we have just sketched, properties like the expansion rate and temperature of each inflated bubble can vary randomly from region to region. However, it is also possible for the strengths and number of low-energy forces of Nature to vary. It is even possible for the number of dimensions of space which have expanded to large size to be different from region to region. We know that we cannot produce the known varieties of organized biochemical complexity if the strengths of forces change by relatively small amounts, or in dimensions other than three because of the impossibility of creating chemical or gravitational bound states.

The possibility of these random variations arises because inflation is ended by the decay of some matter field satisfying. This corresponds to the field evolving to a minimum in its self-interaction potential. If that potential has a single minimum then the characteristic physics that results from that ground state will be the same everywhere. But if the potential has many minima (for example like a sine function) then each minimum will have different low-energy physics and different parts of the universe can emerge from inflation in different minima and with different effective laws of interaction for elementary particles. In general, we expect the symmetry breaking which chooses the minima in different regions to be independent and random.

Contributed by: Dr. John Barrow