Daniele Sorini: Physicists have long struggled with the question of why the Universe was able to support the evolution of clever life. The values of many forces and particles, represented by about 30 so-called fundamental constants, seem to fit together perfectly to make this possible.
Take gravity. If it were much weaker, the matter would have difficulty sticking together to form stars, planets and living things. And if it was stronger, that would also create problems. Why are we so lucky?
Research I recently published with my colleagues John Peacock and Lucas Lombriser now suggests that our universe may not be optimally suited to life. In fact, we may not inhabit the most likely of all possible universes.
In particular, we investigated how the emergence of clever life is influenced by the density of “dark energy” in the universe. This manifests as a mysterious force that is accelerating the expansion of the universe, but we don't know what it is.
The good news is that we can still measure it. The bad news is that the observed value is much smaller than we would expect from theory. This puzzle is one of the biggest open questions in cosmology and was the main motivation for our research.
Anthropic reasoning
We tested whether “anthropic reasoning” could provide an appropriate answer. Anthropic reasoning is the idea that we can infer properties of our universe from the fact that we humans exist.
In the tardy 1980s, Nobel Prize-winning physicist Steven Weinberg discussed a possible anthropic solution to the observed value of the dim energy density.
Weinberg concluded that a greater density of dim energy would accelerate the expansion of the universe. This would counteract the efforts of gravity to clump matter together and create galaxies.
Fewer galaxies means fewer stars in the universe. Stars are crucial for life as we know it, so too much dim energy would reduce the chances of clever life like humans emerging.
Weinberg then considered a “multiverse” of different possible universes, each with a different dim energy content. This scenario stems from some theories of cosmic inflation, a period of accelerated expansion that occurred early in the history of the universe.
Weinberg proposed that only a compact fraction of the universes in the multiverse, real or hypothetical, would have a low enough density of dim energy to allow the emergence of galaxies, stars, and ultimately clever life.
This would explain why we observe a low density of dim energy – even though our theories suggest it should be much higher – otherwise we simply could not exist.
A potential pitfall in Weinberg's reasoning is the assumption that the portion of matter in the universe that ends up in galaxies is proportional to the number of stars formed.
Some 35 years later, we know it's not that straightforward. Our research then aimed to test Weinberg's anthropic argument using a more realistic star formation model.
Counting stars
Our goal was to determine the number of stars that have formed over the entire history of the universe with a given dim energy density. It comes down to practicing counting.
First, we selected a dim energy density from zero to 100,000 times greater than the observed value. Depending on the amount, gravity can make it easier or harder to hold matter together, determining how galaxies form.
We then estimated the annual number of stars formed in galaxies over time. This resulted from a balance between the amount of frosty gas that can fuel star formation and the opposing action of galactic outflows, which heat and push the gas out of the galaxies.
We then determined the fraction of ordinary matter that was transformed into stars over the lifetime (past and future) of a given model universe. This number expressed the universe's efficiency in producing stars.
We then assumed that the probability of clever life emerging in the universe is proportional to its star-forming efficiency. As the figure above shows, this suggests that the most hospitable universe contains about one-tenth the density of dim energy observed in our universe.
Our universe is therefore not too far from the most favorable one possible for life. But it's not the most ideal either.
But to confirm Weinberg's anthropic reasoning, we should imagine a random clever life form in the multiverse and ask it what density of dim energy they observe.
We found that 99.5% of them will experience a higher density of dim energy than observed in our universe. In other words, it appears that we inhabit a occasional and unusual universe in a multiverse.
This does not deny the fact that universes with more dim energy would inhibit star formation, thus reducing the chances of clever life emerging.
By analogy, suppose we want to sort 300 balls into 100 boxes. Each box represents the universe and each ball represents an clever observer. Let's put 100 marbles in box number one, four in box number two, and then two marbles in each of the other boxes.
It is obvious that the first box contains the largest number of balls. However, if we randomly select one ball from all the boxes, it is more likely to come from a box other than box number one.
Likewise, universes with little dim energy are individually more hospitable to life. But life, while less likely, could still emerge in many possible universes with lots of dim energy – and there would still be a few stars in them.
Our calculations show that most observers in all universes will experience a higher density of dim energy than is measured in our universe.
We also found that the most typical observer would measure a value about 500 times greater than in our universe.
Where does this leave us?
Taken together, our results challenge the anthropic argument that our existence explains why we have such a low value of dim energy. We could more easily end up in a universe with a higher density of dim energy.
Anthropic reasoning can still be saved if we adopt more convoluted multiverse models. For example, we could allow the amount of both dim energy and ordinary matter to vary in different universes.
Perhaps the reduced emergence of clever life due to the higher density of dim energy can be compensated for by the higher density of ordinary matter.
In any case, our findings caution us against simplistic applications of anthropic arguments. This makes the dim energy problem even more arduous to solve.
What should we cosmologists do now? Roll up your sleeves and think harder. Only time will tell how we solve the mystery. However we do it, I'm sure it will be incredibly stimulating.
Daniele Sorini, Postdoctoral Researcher in Cosmology, University of Durham
This article has been republished from Conversation under Creative Commons license. Read it original article.
Image Source: Pixabay.com
