Physicists have long grappled with the question of why the universe was able to support the evolution of intelligent life. The values of the many forces and particles, represented by some 30 so-called fundamental constants, all seem to line up perfectly to enable it.
Take gravity. If it were much weaker, matter would struggle to clump together to form stars, planets and living beings. And if it were stronger, that would also create problems. Why are we so lucky?
Research that I recently published with my colleagues John Peacock and Lucas Lombriser now suggests that our universe may not be optimally tailored for life. In fact, we may not be inhabiting the most likely of possible universes.
We particularly studied how the emergence of intelligent life is affected by the density of “dark energy” in the universe. This manifests as a mysterious force that speeds up the expansion of the universe, but we do not know what it is.
The good news is that we can still measure it. The bad news is that the observed value is way smaller than what we would expect from theory. This puzzle is one of the biggest open questions in cosmology, and was a primary motivation for our research.
Anthropic reasoning
We tested whether “anthropic reasoning” may offer a suitable answer. Anthropic reasoning is the idea that we can infer properties of our universe from the fact that we, humans, exist. In the late 80s, physics Nobel laureate Steven Weinberg discussed a possible anthropic solution for the observed value of the dark energy density.
Weinberg reasoned that a larger dark energy density would speed up the universe’s expansion. This would counteract gravity’s effort to clump matter together and form galaxies. Fewer galaxies means fewer stars in the universe. Stars are essential for the emergence of life as we know it, so too much dark energy would suppress the odds of intelligent life such as humans appearing.
Weinberg then considered a “multiverse” of different possible universes, each with a different dark energy content. Such a scenario follows from some theories of cosmic inflation, a period of accelerated expansion occurring early in the universe’s history.
Weinberg proposed that only a tiny fraction of the universes within the multiverse, whether real or hypothetical, would have a sufficiently small dark energy density to enable galaxies, stars and, ultimately, intelligent life, to appear. This would explain why we observe a small dark energy density – despite our theories suggesting it should be much larger – we simply could not exist otherwise.
Credit: Courtesy of Oscar Veenema, former undergraduate student at Durham University, now PhD student at Oxford University, CC BY-SA
A potential pitfall in Weinberg’s reasoning is the assumption that the fraction of matter in the universe that ends up in galaxies is proportional to the number of stars formed. Some 35 years later, we know that it is not that simple. Our research then aimed at testing Weinberg’s anthropic argument with a more realistic star formation model.
Counting stars
Our goal was to determine the number of stars formed over the entire history of a universe with a given dark energy density. This boils down to a counting exercise.
First, we picked a dark energy density between zero and 100,000 times the observed value. Depending on the amount, gravity can hold matter together more or less easily, determining how galaxies can form.
Next, we estimated the yearly amount of stars formed within galaxies over time. This followed from the balance between the amount of cool gas that can fuel star formation, and the opposing action of galactic outflows that heat up and push gas outside galaxies.
We then determined the fraction of ordinary matter that was converted into stars over the entire lifetime (past and future) of a certain universe model. This number expressed the efficiency of that universe at producing stars.
Credit: Image readapted from D. Sorini, J. A. Peacock, L. Lombriser, in Monthly Notices of the Royal Astronomical Society, Volume 535, Issue 2, Pages 1449–1474. Source: https://doi.org/10.1093/mnras/stae2236, CC BY-SA
We then assumed that the likelihood of generating intelligent life in a universe is proportional to its star formation efficiency. As the figure above shows, this suggests that the most hospitable universe contains about one-tenth of the dark energy density observed in our universe.
Our universe is thus not too far from the most favourable possible for life. But it also isn’t the most ideal.
But to validate Weinberg’s anthropic reasoning, we should imagine picking a random intelligent life form in the multiverse, and ask them what dark energy density they observe.
We found that 99.5% of them would experience a larger dark energy density than observed in our universe. In other words, it looks like we inhabit a rare and unusual universe within the multiverse.
This does not contradict the fact that universes with more dark energy would suppress star formation, hence reducing the chances of forming intelligent life.
CC BY-SA
By analogy, suppose we want to sort 300 marbles into 100 boxes. Each box represents a universe, and each marble an intelligent observer. Let us put 100 marbles in box number one, four in box number two and then two marbles in all other boxes. Clearly, the first box contains the single largest number of marbles. But if we pick one marble at random from all boxes, it is more likely to come from a box other than number one.
Likewise, universes with little dark energy are individually more hospitable for life. But life, although more unlikely, can still spawn in the many possible universes with abundant dark energy too – there will still be a few stars in them. Our calculation finds that most observers among all universes will experience a higher dark energy density than is measured in our universe.
Also, we found that the most typical observer would measure a value about 500 times larger than in our universe.
Where does that leave us?
In conclusion, our results challenge the anthropic argument that our existence explains why we have such a low value of dark energy. We could have more easily found ourselves in a universe with a larger dark energy density.
Anthropic reasoning may still be salvaged if we adopt more complex multiverse models. For example, we could allow for the amount of both dark energy and ordinary matter to vary across different universes. Perhaps, the reduced spawning of intelligent life due to a higher dark energy density might be compensated by a higher density of ordinary matter.
In any case, our findings warn us against a simplistic application of anthropic arguments. This makes the dark energy problem even harder to grapple with.
What should we cosmologists do now? Roll up our sleeves and think harder. Only time will tell how we solve the puzzle. However we will do it, I am sure it will be incredibly exciting.
