Inflation Theory Takes a Little Kick in the Pants
Written by Nancy Atkinson
Inflation theory proposes that the universe underwent a period of exponential expansion right after the Big Bang. One of the key predictions of inflation theory is the presence of a particular spectrum of "gravitational radiation"—ripples in the fabric of space-time that are really hard to detect but thought to exist. But a team of researchers has now found that gravitational radiation can be produced by a mechanism other than inflation. So this type of radiation, if eventually detected, won’t provide the conclusive evidence for inflation theory that was once was thought to be a certainty.
"If we see a primordial gravitational wave background, we can no longer say for sure it is due to inflation," said noted astronomer Lawrence Krauss, from Case Western Reserve University.
Inflation theory first was proposed by cosmologist Alan Guth in 1981 as a means to explain some features of the universe that had previously baffled astronomers, such as why the universe is so close to being flat and why it is so uniform. Today, inflation remains the best way to theoretically understand many aspects of the early Big Bang universe, but most of the theory’s predictions are somewhat vague enough that even if the predictions were observed, they probably wouldn’t provide a clear-cut confirmation of the theory.
But gravitational radiation was considered one of the key predictions of inflation theory, and detection of this spectrum was regarded among physicists as "smoking gun" evidence that inflation did in fact occur, billions of years ago.
Gravitational radiation is a prediction of Einstein's Theory of General Relativity. According to the theory, whenever large amounts of mass or energy are shifting around, it disrupts the surrounding space-time and ripples emanate from the region where the shift occurs. These ripples aren’t easily detected, but there is one experiment designed to look directly for this radiation, the Laser Interferometer Gravitational Wave Observatory (LIGO) in Livingston, Louisiana. The upcoming Planck Mission, set to launch in 2009 will look for it indirectly by looking at the cosmic microwave background.
Until now it was widely believed that detecting gravitational radiation in the form of polarized light from the CMB would confirm inflation theory, since it was thought inflation would be the only way this radiation could be produced. But Krauss and his team have raised the issue of whether this radiation can be unmistakably tied to inflation.
Krauss’s team proposes that a phenomenon called "symmetry breaking," can also produce gravitational radiation. Symmetry breaking is a central part of fundamental particle physics, where a system goes from being symmetrical to a low energy state that is not symmetrical. Krauss’s explanation is that a "scalar field" (similar to an electric or magnetic field) becomes aligned as the universe expands. But as the universe expands, each region over which the field is aligned comes into contact with other regions where the field has a different alignment. When that happens the field relaxes into a state where it is aligned over the entire region and in the process of relaxing it emits gravitational radiation.
This is all fairly confusing, but the sweetened condensed version is that if gravitational radiation is ever detected, that event won’t necessarily verify inflation theory. Therefore, whether inflation theory can ever be confirmed remains to be seen.
Krauss’s paper "Nearly Scale Invariant Spectrum of Gravitational Radiation from Global Phase Transitions" is published in the Aprill 2008 Physical Review Letters.
Original News Source: Case Western Reserve University press release
Filed under: Physics
April 17th, 2008 at 12:03 am
As far as I know symmetry breaking and inflation do not exclude each other, in fact the energy density of the universe is closely linked to the expansion rate. So if they find the right wavelength of gravitational waves, isn't it a false dichotomy to say either or?
April 17th, 2008 at 12:24 am
i've never been a huge fan of inflation theory. i wouldn't mind seeing it disproved in my lifetime.
hopefully this is one more nail in the coffin.
April 17th, 2008 at 2:36 am
I guess the connection between inflation and symmetry break(s) (or phase transition(s)) in the (very) early big bang universe should be found in the correct formulation of quantum gravity, perhaps quantized general relativity if field theory, which it need not be. If the correct quantum gravity theory is a simultaneous far action theory (like Newtonian gravity and inertia and Amperian electromagnetic theory), a large chunk of established current physics theory must (at least) be reformulated (if not reduced to an approximation of simultaneous far action theory as the basic structute of reality). Wait and see!
April 17th, 2008 at 6:31 am
They are not arguing against inflation theory as much as arguing the proper way to measure it. The static state model, the Big Bang model and string theory all require inflation to be valid.
Richard Wigmans has proposed that some of what we are seeing is due to degenerate neutrinos. The question he raises is obvious. Since degenerate electrons fail to shift energy levels and become locked in such saturated energy levels, do degenerate neutrinos fail to oscillate? Did primordial neutrinos settle at the center of masses of galaxy clusters? Do they resist gravity just as degenerate electrons do as well and, by doing so, do they push away at the galaxies surrounding them instead of holding a dying star together, contributing to some of the expansion we are seeing?
I am sure that even that will be falsified but it does point to the fact that we have no present lock on an experiment for spatial inflation. Future experiments will likely uncover better experiments with better equipment.
April 17th, 2008 at 6:44 am
ooohhhh, I love it when you guys talk way over my head!
(understanding little, but still engrossed!)
April 17th, 2008 at 7:54 am
Steady on, Peter K! you may need to lie down in a dark room for a bit. or have a cold shower.
Anyways, isn't this EXACTLY what science is all about? a constant process of checking, understanding & re-evaluation of what we know/find out/ can explain?
I'd like to see where it leads
April 17th, 2008 at 7:59 am
So, nobody's found this stuff but even if we did find the ripple [and not drink it], it can't prove inflation. Ok. Got it. And 'ya all get paid for writing this dribble!
April 17th, 2008 at 8:44 am
You describe
"… a "scalar field" [as being] (similar to an electric or magnetic field)…"
I have always understood a scalar field to mean a "field of scalar quantities" similar to a temperature or pressure field. Both the electric and magnetic field are fields of vector quantities i.e. they have direction.
April 17th, 2008 at 1:12 pm
I wouldn't worry too much about gravitational radiation because it does not exist. LIGO nor LISA will never detect it so inflation theory is safe.
April 17th, 2008 at 1:19 pm
Yes, both the magnetic and electric fields are vector fields, not scalar.
Interestingly, the electric field can be described by the gradient of scalar field, in the absence of time varying fields, called the electric potential V. Although the magnetic field always needs the curl of vector potential field (named A usually) to be described.
April 17th, 2008 at 2:12 pm
ff43, I'm happy that fundamental potential field theory hasn't changed since my retirement from about 25 years of studying it in the context of exploration geophysics! However, accepting that a scalar field can have no direction, then the statement
"…that a "scalar field" … becomes aligned as the universe expands. But as the universe expands, each region over which the field is aligned comes into contact with other regions where the field has a different alignment…" seems to be rather a problem.
If vector (or some tensor of higher order) fields are involved, presumably they could have differing orientation in different regions of space-time. However, I see no description of what these vector fields are, nor how they could be detected. Presumably they should reveal their presence by some kind of cosmic anisotropy.
April 17th, 2008 at 3:09 pm
Oh, I didn't mean to say that the scalar field that are talking about here is the underlying of some vector field. I couldn't find any mention of such case in the paper where this news comes from either (although it is way over what I can actually understand, so maybe it does). I'm not entirely sure what this field is supposed to be.
In any case, what I think I understand from this scalar fields aligning with each other, it just means that they take the same value at that point. For instance, you mention how temperature can be thought of as a scalar field. So if you were to place two objects with different temperatures in contact, the temperatures would equalize there (and then to the rest of both objects). So in a sense they get aligned. My guess is that they mean something similar to this, although I really don't know.
April 17th, 2008 at 3:15 pm
Also, I forgot to add, that by taking the gradient of a scalar field (i.e. how the field's value changes at each point relative to the next) one can extract a vector field that tells you how the values of the scalar field change, so in a sense they do have an inherent direction of variation.
April 17th, 2008 at 7:13 pm
Thanks ff43.
I was confused by the comparison of a scalar field with an electric or magnetic field. I tracked down the paper (I think) at "arXiv:0712.0778v1 [astro-ph] 5 Dec 2007" for those of us without institutional access to the journals. After skimming it I get the impression that the phase transition he is referring to is one similar to the breaking of symmetry that occurs at the electroweak transition (or perhaps the strong nuclear transition).
If I understand him correctly, (and my math is not what it once almost was) one might compare this (very roughly) to the phase transition that would occur at an expanding crystallization front if one dropped a seed crystal into a supercooled liquid. The expanding crystallization front would generate a pressure wave that would propagate away from that crystallizing front. In an analogous manner, gravitational waves would be generated by symmetry breaking in the early universe.
April 20th, 2008 at 6:23 am
Well, does "Dark Energy" play a role at all?
April 30th, 2008 at 6:13 am
In an inflationary process, just what fields would the new universe be expanding over. This presumes some archaic field presence from before the big bang or big miss if we are to believe Hawking's latest or the instantaneous generation of those fields at the event. In any case, it would seem that these fields of varying alignments would produce variously polarized waves of different frequencies and amplitudes, depending on the field being expanded over rather than a consistent radiation. Also, despite all the theory, no one has detected any gravity waves at all, much less those with some particular characteristics, so this whole discussion seems more than a bit premature.