Deflating the Inflation
I'm reading Roger Penrose's "The Road to Reality"--a book every physicist should read. There are very few physicist with such depth of insight and the ability to "say how it is". So Penrose is now my second favorite after Feynman. (I still think Penrose's ideas about human consciousness are embarrassing, but that's a different topic altogether.)
I read Penrose's arguments against cosmic inflation and they make a lot of sense to me.
Inflation is supposed to "solve" some cosmological problems. For instance, why is the universe so homogeneous and flat.
Look ten billion light years to the left, then the same distance to the right, and you'll be amazed at how similar the two views are. In particular, the background radiation coming from both areas has almost identical temperature. Why is it so strange? Well, normally when two object have the same temperature, we assume that they must have been in contact with each other at some point--either directly or indirectly. The two areas of cosmos we are talking about could never have been in contact, because even the light from one of them didn't have the time to reach the other during the lifetime of the universe. The light has barely had enough time to reach us, who are in between the two of them.
Thermal equilibrium spreads much slower than light!
So, either these two areas started out at the same temperature (by coincidence? by design?), or we are wrong about them never having had time to exchange heat.
The inflation theory choses the second option. It postulates that there was a very short period of time, right after the Big Bang, during which the expansion of the universe was exponential (and not hyperbolic, parabolic, or elliptic--as the Einstein's equations would suggest, without the cosmological constant). The trick with exponential expansion is that it gets very fast, but it starts out real slow. It was during that initial slow period when tiny patches of the cosmos had a chance to reach internal thermal equilibrium. Immediately after that, they were blown up to enormous sizes--our whole visible universe originated from just one such small patch.
It all sort of makes sense; so much in fact that the inflation theory became part of the canon of cosmology, even though the actual cause of inflation is not known. (Sure, there are speculations having to do with spontaneous symmetry breaking in some yet unspecified grand unification theory. All those are way beyond our mathematical reach.)
What Penrose points out is that this "solution" substitutes one conundrum with another, even bigger. The bigger conundrum is, why was the entropy of the universe so unbelievably low at Big Bang?
We live in a universe where the Second Law of Thermodynamics rules. Entropy is always increasing in leaps and bounds.
You have to understand that entropy is not unbounded. It increases only until the system reaches the state of the equilibrium. At that point the Second Law of Thermodynamics stops working. The fact that it still works proves that the universe started very-very-very-far from the equilibrium. It was very precariously balanced on the tip of an enormous needle.
The local thermal equilibriums postulated by the inflation could have only been reached by increasing the entropy (the smoothing out of differences increases entropy). It means that the inflation theory requires the universe to start with even lower entropy. The enormous needle gets even taller!
What Penrose really uncovers is how subjective science is, at least at this highly speculative level. The truth is that we have no way of deciding what the "natural" state of the nascent universe should have been. Apparently, our bias is that a uniform state in not natural. We have to explain uniformity. We don't have to explain non-uniformity. Why is it so?
Are we biased because we were brought up in the universe where the Second Law of Thermodynamics plays such an important role--in fact, making life possible?
I read Penrose's arguments against cosmic inflation and they make a lot of sense to me.
Inflation is supposed to "solve" some cosmological problems. For instance, why is the universe so homogeneous and flat.
Look ten billion light years to the left, then the same distance to the right, and you'll be amazed at how similar the two views are. In particular, the background radiation coming from both areas has almost identical temperature. Why is it so strange? Well, normally when two object have the same temperature, we assume that they must have been in contact with each other at some point--either directly or indirectly. The two areas of cosmos we are talking about could never have been in contact, because even the light from one of them didn't have the time to reach the other during the lifetime of the universe. The light has barely had enough time to reach us, who are in between the two of them.
Thermal equilibrium spreads much slower than light!
So, either these two areas started out at the same temperature (by coincidence? by design?), or we are wrong about them never having had time to exchange heat.
The inflation theory choses the second option. It postulates that there was a very short period of time, right after the Big Bang, during which the expansion of the universe was exponential (and not hyperbolic, parabolic, or elliptic--as the Einstein's equations would suggest, without the cosmological constant). The trick with exponential expansion is that it gets very fast, but it starts out real slow. It was during that initial slow period when tiny patches of the cosmos had a chance to reach internal thermal equilibrium. Immediately after that, they were blown up to enormous sizes--our whole visible universe originated from just one such small patch.
It all sort of makes sense; so much in fact that the inflation theory became part of the canon of cosmology, even though the actual cause of inflation is not known. (Sure, there are speculations having to do with spontaneous symmetry breaking in some yet unspecified grand unification theory. All those are way beyond our mathematical reach.)
What Penrose points out is that this "solution" substitutes one conundrum with another, even bigger. The bigger conundrum is, why was the entropy of the universe so unbelievably low at Big Bang?
We live in a universe where the Second Law of Thermodynamics rules. Entropy is always increasing in leaps and bounds.
You have to understand that entropy is not unbounded. It increases only until the system reaches the state of the equilibrium. At that point the Second Law of Thermodynamics stops working. The fact that it still works proves that the universe started very-very-very-far from the equilibrium. It was very precariously balanced on the tip of an enormous needle.
The local thermal equilibriums postulated by the inflation could have only been reached by increasing the entropy (the smoothing out of differences increases entropy). It means that the inflation theory requires the universe to start with even lower entropy. The enormous needle gets even taller!
What Penrose really uncovers is how subjective science is, at least at this highly speculative level. The truth is that we have no way of deciding what the "natural" state of the nascent universe should have been. Apparently, our bias is that a uniform state in not natural. We have to explain uniformity. We don't have to explain non-uniformity. Why is it so?
Are we biased because we were brought up in the universe where the Second Law of Thermodynamics plays such an important role--in fact, making life possible?