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The Anthropic Principle and
Cosmic Inflation

3. Inflation Theory and the Multiverse

One way to express this notion of many different local domains, each with their own physical laws and constants is with the idea of the multiverse.[5] The idea of the multiverse is not a wild philosophical fancy on the part of some cosmologists, arrived at after a heavy night of drinking. It arises from the cosmologists' models of the constituents of the universe. One of those constituents is dark energy.

Dark energy is the energy of the vacuum; of empty space. It works in opposition to the pulling effects of gravity. The observed expansion of our universe is occurring at an accelerating rate as a result of the action of dark energy. This accelerating expansion is a natural consequence of the fact that the density of dark energy stays constant with time while the density of matter declines as the universe expands. It is this constancy that leads modern cosmologists to identify dark energy with Einstein's cosmological constant. The history of the universe has passed the point at which the density of matter is greater than the density of dark energy. The effects of dark energy compared with that of matter have now tipped in dark energy's favour.

Diagram 1 – Evolution of density of matter and dark energy[6]

Diagram charting density of matter and dark energy over time, showing crossover point at current time in the history of the universe

The density of dark energy is observed to be 6 x 10−27 kg m−3. However, this value is much smaller than the density expected from quantum mechanical calculations by many orders of magnitude. Using quantum mechanics and Einstein's mass–energy equivalence (e = mc2), physicists calculate an expected Planck scale vacuum density of 10100 kg m−3. By this reckoning, the masses of elementary particles turn out also to be much lower than expected. It is the solution to these two problems that led physicists to the possibility of the multiverse.

Physicists proposed an early period of inflation just after the birth of the universe to solve another problem in cosmology; the horizon problem. The temperature of the cosmic microwave background (CMB) is highly uniform (varying by only 1 part in 100,000), yet regions of the CMB were so far apart during the time of the early universe that even light was not fast enough to travel from one such region to the other. This apparent lack of causal connectedness between regions is solved by positing an early period of rapid inflation in which the regions were in causal contact prior to the period of inflation. After the initial inflationary period, it is thought, the vacuum energy dissipates and drops to the much lower level observed today.

As it turned out, inflation theory also accounted very accurately for the quantitative irregularities in the CMB and for the seeds of structure in the early universe that led to the large scale structure we see today. Taking account of inflation, the universe turns out much older than cosmologists thought. The universe did not arise from a singularity 13.7 billion years in the past, as was supposed. The universe was accelerating exponentially prior to this time; prior to what was thought to be the big bang. The puzzle now is to work out how the vacuum energy density can change with time.

Diagram 2 – History of the expansion of the universe

Diagram charting the size of our universe, showing exponential expansion through big bang period

The most promising answer to the puzzle borrows from Peter Higgs work in the 1960s on the Higgs field. Just as the Higgs field allows for different energy levels for the vacuum energy, a newly proposed inflaton field allows the vacuum density to change dynamically.

The thing with inflation is that it arises spontaneously from random quantum fluctuations in the vacuum. This process of blowing up a small region of space can happen many times in many different places. It is this blowing up of a very small quantum region into a massive bubble that gives us a new universe. Each of these bubbles is physically isolated from the others and has its own physical laws and constants. Each has its own value for the minimum vacuum energy density, the Vacuum State. In most bubbles, the value would be extremely high. However, quite independently of cosmological considerations, physicists using string theory calculate the number of possible minimum values for the energy density to be in the order of 10500. Some of these minimum values approach zero.

Diagram 3 – Possible minimum values of vacuum density and our universe

Diagram charting possible values of vacuum density in various universes, accourding to string theory, and the low value in our own universe.

Our universe is a universe with just such a low value for the vacuum density. In 1987, Steven Weinberg (the author of the Standard Model of elementary particles) used the anthropic argument to show why we don't live in a universe with a large vacuum density. He pointed out that if the vacuum density is not less than a specific threshold value, gravity will not sufficiently counteract the inflationary force to form the universe's large scale structures (i.e., galaxy structures).

Cosmologists cannot test inflation theory by directly conducting experiments in other universes as these are forever beyond our reach. However, they are able to test the consequences of the theory. In 1979, the Soviet physicist, Alexei Starobinsky, realized that the early inflationary period did not only modulate the density of matter in the young universe. He saw that it also modulated the gravitational field. From this realisation, he predicted the existence of relic gravitational waves left over from the early inflationary period. However, these gravitational waves are not easy to detect. How can they be detected? Starobinsky predicted that these gravitational waves will leave their imprint in the form of B-mode polarisation of light on the last scattering surface (the CMB) some 380,000 years after the end of the period of inflation.

For years, cosmologists have been searching for gravitational waves. On March 17th 2014, the BICEP2 research team, using the telescope mounted at the South Pole, announced that they had detected relic gravitational waves.[7] The results are currently being debated as measurements from the Plank satellite indicate that polarisation from dust in our own Milky Way galaxy may be muddying the results. We will need to wait for confirmation. These are indeed very exciting times.

In this essay, I considered the view that our universe has been fine-tuned for life to evolve. Carter's Anthropic Principle shows us how the fact that we are able to observe the universe constrains the kinds of universe we could observe. Even so, some scientists misconstrued the Anthropic Principle to construct elaborate teleological theories. I then explored how the application of quantum mechanics and string theory to the evolution of the early universe provides evidential support for the notion of a period of hyper-inflation. Tantalizingly, the theory underpinning inflation entails that we live in a multiverse in which the conditions for the evolution of life vary from region to region. In this way, we have come back full circle to the Anthropic Principle and a testable empirical theory that answers the question of why we find the universe the way it is.

Footnotes

  1. [5] Some theorists use the term 'bubble universe' or 'pocket universe'.
  2. [6] All diagrams sourced from Slides for The Anthropic Principle in Cosmology.pdf, Alasdair Richmond, University of Edinburgh.
  3. [7] For the latest BICEP2 research results, refer to http://bicepkeck.org/

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