Cosmological Constant Conundrum

I was browsing the online archives of physics papers today when I stumbled across an intriguing paper by Abraham Loeb of Harvard.

Loeb claims that a search for planets in dwarf galaxies could test, and possibly debunk, a common explanation for the puzzling measurement of the cosmological constant, which drives the accelerating expansion of the universe.

Einstein initially proposed the cosmological constant, as a part of his theory of general relativity, to explain why the universe appeared to be static. When Edwin Hubble showed that the universe is actually expanding, Einstein dropped the constant from his theory, calling it the greatest blunder of his career.

It turned out later that Einstein might have been on the right track for the wrong reasons. The universe is not simply expanding, but accelerating as well. As a result, physicists resurrected the constant and found that it must have a value around 0.7 to match our observations of the universe.

Although it is a tidy explanation of the universe’s accelerating expansion, the observed value is problematic. Some promising theories predict that the constant should be thousands of times larger than it seems to be, while others require it to be exactly zero. None seem to predict the value we actually measure (although string theory may offer a mechanism that allows the constant to decrease as the universe goes through many Big Bang and Big Crunch cycles).

One popular, though controversial, explanation for the measured value stems from the idea our universe is only one of many in an enormous multiverse, and that the cosmological constant may take on different values in different universes. According to the anthropic argument, most values of the constant lead to sterile, lifeless universes, while only values near the one we see are conducive to life. We measure a small cosmological constant because we wouldn’t be around to measure a larger one, regardless of how unlikely a small constant is among the nearly countless possible universes. It’s a pat argument and difficult to argue with, considering the fact that we can’t visit other universes to check it.

Loeb, however, thinks we can do the next best thing right here in our universe by looking for planets in nearby dwarf galaxies. The conditions in dwarf galaxies, when they formed in the early universe, were apparently similar to those that would exist in universes with cosmological constants thousands of times higher than ours. If we find plentiful planets in dwarf galaxies, some of which are likely to be conducive to life, then Loeb says that we can be 99.9% confident that anthropic calculations of the cosmological constant are meaningless.

Loeb points out at the end of his paper that there might be an added benefit to finding planets in old dwarf galaxies “If the anthropic argument turns out to be wrong,” he writes, “and intelligent civilizations are common in nearby dwarf galaxies, then the older more advanced civilizations among them might broadcast an explanation for why the cosmological constant has its observed value.”

*Personal opinion warning*

I think Loeb’s paper could have profound implications for theories that predict multiple universes with random cosmological constant values. (String theory, for instance, suggests that there are as many as 10^500 possible universes – that’s a ten with 500 zeroes after it – with all sorts of values for the fundamental constants).

According to multiverse theories, our universe is already unusual because the other constants (such as those controlling the strength of gravity, electromagnetic interactions, etc.) are finely tuned to support life. If life can exist in universes with high cosmological constants, then we can’t use the anthropic argument to explain ours. How much more unlikely must it be, then, for us to have an unusual value for the cosmological constant? (I would say N/10^500, where N is the number of habitable universes with cosmological constant of about 0.7. I have no idea what N should be, though.)

I may be making an illogical leap here, but if it turns out that we can’t make a case for an anthropically determined value for the cosmological constant, then any theory that says our constant is unlikely is equally unlikely to be true.

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