Authors: Fred C. Adams, Katherine R. Coppess, Anthony M. Bloch
First author’s institution: University of Michigan
Status: To be submitted to JCAP
The artist’s conception to the right shows a lonely planet, wandering through space. Planets like these could be habitable in other Universes, say Adams, Coppess & Bloch.
Any teeny-tiny variations in the density of matter and radiation that existed, fractions of a second after the big bang, eventually snowballed into galaxy clusters and voids. The amplitude of the primordial density fluctuations, Q, has played a fundamental role in designing the Universe of today.
Q describes how big the primordial fluctuations were. Larger Q means bigger fluctuations; i.e. the empty space was more empty and the dense space was more dense. Tiny differences in the amplitude of density fluctuations at the beginning of time would manifest as huge differences in the Universe of today. This paper explores how exactly how increasing Q affects the life-supporting conditions in other Universes.
If Q is too large, galaxies are very dense; the average distance between stars decreases and stellar collisions become much more common. Figure 1 shows how the fraction of planetary systems that survive changes as you increase Q. The value of Q in our Universe is 10-5. The different lines show the result for galaxies of different masses, where M = 1010 − 1014 Msun, from lower left to upper right.
An increased number of collisions isn’t the only factor to affect habitability. If the average distance between stars were to decrease, starlight would contribute a significant fraction of the radiation received by a planet until, eventually, the stars would outshine the planet’s own Sun! At some value of Q, the starlight would get so intense that planets orbiting stars would be blasted with radiation and life wouldn’t have a chance.
So, if Q is too high, stellar collisions and lethal starlight ruin the chances of developing life. However, there is another opportunity for galaxies with a higher Q than ours. If stars are just the right distance apart that all the free-floating planets in the galaxy are bathed in gentle, warming radiation, a planet won’t even need a host star to be in the ‘habitable zone‘! There could potentially be millions of free-floating, habitable planets, heated purely by starlight. These planets would need to be far enough from the galactic centre that they avoid collisions and extreme radiation, but not so far that they aren’t heated enough bt starlight. They need to lie in, what the authors call, the ‘Galactic Habitable Zone’.
Figure 3 shows the fraction of planetary systems that are within the galactic habitable zone as a function of X, where X is a sort-of proxy for Q. It’s a composite parameter which is described by the stellar density, the characteristic scale of the galaxy and the average luminosity of all the stars in the galaxy. The dashed line shows the fraction of habitable planets expected in our own galaxy. So Universes producing galaxies with f > 0.1 will be more habitable than ours! In other words, many parallel Universes could be even more habitable than our own!