The cosmological constant is like a knob that affects how quickly the universe's expansion is accelerating or decelerating. As a rule of thumb, the more positive the constant is the faster the expansion accelerates and the more negative the more it decelerates. If it is zero, and there is just the right amount of matter, the universe just stays flat and we never experience a rapid acceleration or deceleration in expansion.
First, all positive values are bad. Now, what does this have to do with life? It turns out a positive cosmological constant, like our own, actually dilutes matter and prevents a lot of gravitational collapse making our universe less likely for life than if the constant were not positive. From the paper:
The reason is that a positive cosmological constant gives a repulsion between separate particles that reduce the ordinary gravitational attraction and leads to less gravitational condensation of matter. Therefore, other factors being equal, any positive cosmological constant decreases the fraction of baryons that condense to form galaxies and other structures that eventually form living substructures.
As an immediate consequence, no positive value of the cosmological constant (such as the observed value Λ) can maximize the fraction of baryons in lifeBut wouldn't a negative value also be bad? Yes, because if the value is too negative the universe recollapses and life doesn't have time to form. Page keeps this in mind while calculating the best value to find that Goldilocks region that is most optimal for life. That said, he does find that the optimal values for life in the universe are slightly negative on the order of Λ ~ -10-120.
So God created a Multiverse? Interestingly enough Page is very religious and so does not conclude this is evidence against God but actually evidence that God must have created a multiverse where each pocket universe has a different cosmological constant like most modern cosmology theories predict. From the paper:It might be appropriate to note that although this paper has focused on the scientifically testable question of whether the constants of physics maximize a particular measure for life, it obviously also has theological implications. It could be taken as negative evidence for theists who expect God to fine tune the constants of physics optimally for life. However, for other theists, such as myself, it may simply support the hypothesis that God might prefer a multiverse as the most elegant way to create life and the other purposes He has for His Creation.I for one am a big fan of the multiverse because all modern cosmological theories with inflation lead to a multiverse.
And religion aside, given our cosmological constant is such a bizarre value and currently seems to be best explained by multiverse models I will agree with Page that the cosmological constant seems to hint at a multiverse. (Which is why many respected theoretical physicists suggest the peculiar value for the cosmological constant is the best evidence so far for crazy multiverse models like the string landscape.)
Thoughts?
Don N. Page (2011). Evidence Against Fine Tuning for Life E-Print arXiv: 1101.2444v1
I think there are many theological questions, especially in Mormanism, where a multiverse is supportive.
ReplyDeleteI am currently reading "The Day We Found the Universe" by Marcia Bartusiak. Just last night I was reading about Einstein working out general relativity and adding in the cosmological constant to hold everything in the Universe still. I had always thought that constant was to balance things gravitationally. I didn't know until about a year ago that it describes an actual force other than gravity, or does it? In relation to general relativity and the cosmological constant, are gravity and dark energy interchangeable? It's just a number, after all, that plays into the curvature of space time?
Stan,
ReplyDelete1. Great an unsolved question. It *looks* like dark energy is the same as a cosmological constant in all the ways we can measure such things thus far. It though is technically still an unknown and all we know for sure is dark energy seems to act a lot like a cosmological constant. Maybe future experiments will shed more light.
2. Whether it balances the universe or not has to do with the matter content of the universe so there are technicalities here that make it confusing at times. In our own universe the matter content + dark energy (assuming it is the cosmological constant) is such that the cosmological constant being positive is causing an acceleration and it it was negative would cause a deceleration.
3. The "force" you refer to come straight from Einstein's GR equations. If you add a cosmological constant to those equations it leads to equations that cause exponential expansion. G_uv = 8*pi*T_uv leads to exponential expansion if T_uv is a cosmological constant.
Fascinating. Other than that I have nothing to add. Very interesting though.
ReplyDeleteI have just replaced this paper at arXiv:1101.2444, and the new version should become publically available in a few hours. Robert Mann, Michael Salem, and Martin Rees convinced me that although reducing the cosmological constant would increase the fraction of baryons condensing into galaxies, that would not necessarily increase the fraction of baryons going into life, so it is still an open question as to whether the cosmological constant has a value to maximize the latter fraction. Don Page
ReplyDeleteDr. Page,
ReplyDeleteThank you for your comment.