Re: [asa] Inflation

From: Rich Blinne <rich.blinne@gmail.com>
Date: Tue Jul 29 2008 - 19:13:23 EDT

On Tue, Jul 29, 2008 at 4:43 PM, gordon brown <Gordon.Brown@colorado.edu>wrote:

> As I was browsing in a library today, I came across the June issue of New
> Scientist. It contained an article that claimed that the inflationary theory
> of Big Bang cosmology was in trouble. It said that a Prof. Wandelt at the
> University of Illinois, a proponent of a cyclic cosmology, had pointed to
> the fact that some predictions of the inflationary theory had failed to be
> detected. One of these was gravitational waves. Do any of you know any more
> about this?
>
> Gordon Brown (ASA member)
>
>
>
> To unsubscribe, send a message to majordomo@calvin.edu with
> "unsubscribe asa" (no quotes) as the body of the message.
>

LIGO still hasn't found any gravitational waves but it still could. Planck
won't launch until 2009 and it could find them indirectly through polarized
CMB. So, it's too early to declare that they have "failed" to detect them.
On the other hand, the problem now is if even if they did find gravitational
waves it wouldn't necessarily be the "smoking gun" for inflation that it
once was thought. See here for the paper:

http://link.aps.org/abstract/PRL/v100/e131302

Here's the press release that accompanied the paper:

http://blog.case.edu/case-news/2008/04/14/gravity

Gravity wave "smoking gun" fizzles, according to Case Western Reserve
University physics researchers

But gravitational waves may be more sensitive probe of early universe
physics than previously thought

A team of researchers from Case Western Reserve University has found that
gravitational radiation—widely expected to provide "smoking gun" proof for a
theory of the early universe known as "inflation"—can be produced by another
mechanism.

According to physics scholars, inflation theory proposes that the universe
underwent a period of exponential expansion right after the big bang. A key
prediction of inflation theory is the presence of a particular spectrum of
"gravitational radiation"—ripples in the fabric of space-time that are
notoriously difficult to detect but believed to exist nonetheless.

"If we see a primordial gravitational wave background, we can no longer say
for sure it is due to inflation," said Lawrence Krauss, the Ambrose Swasey
Professor of Physics and Astronomy at Case Western Reserve.

At the same time the researchers find that gravitational waves are a far
more sensitive probe of new physics near the highest energy scale of
interest to particle physicists than previously envisaged. Thus their work
provides strong motivation for the ongoing quest to detect primordial
gravitational radiation.

Krauss, along with Case Western Reserve colleagues Katherine Jones-Smith, a
graduate student, and Harsh Mathur, associate professor of physics, present
these findings in an article "Nearly Scale Invariant Spectrum of
Gravitational Radiation from Global Phase Transitions" published in Physical
Review Letters this month.

Inflation theory arose in the 1980s 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 universe, but most of its predictions are sufficiently malleable
that consistency with observation cannot be considered unambiguous
confirmation.

Enter gravitational radiation—the key prediction of inflation theory is the
presence of a particular spectrum of gravitational radiation. Detection of
this spectrum was regarded among physicists as "smoking gun" evidence that
inflation did in fact occur, billions of years ago.

In 1992 Krauss, then at Yale, argued that another mechanism besides
inflation could give rise to precisely the same spectrum of gravitational
radiation as is predicted by inflation. The argument given by Krauss in 1992
provided a rough estimate of the spectrum.

Last year Krauss teamed up with Case Western Reserve colleagues,
Jones-Smith, a graduate student in physics, and Mathur, associate professor
of physics, to do a more complete calculation. They found that the exact
calculation predicts the signal to be much stronger than the rough estimate.

Describing their results, Krauss said, "It is shocking and surprising when
you find the answer is 10,000 times bigger than the rough estimate and could
possibly produce a signal that mimics the kind produced by inflation."

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 mass/energy shift.

These space-time ripples, known as gravitational radiation, are
imperceptible on the human scale, but highly sensitive experiments (such as
the Laser Interferometer Gravitational Wave Observatory (LIGO) in
Livingston, La.) are designed precisely to look for such radiation and are
the only hope for detecting them directly.

However, gravitational radiation from the early universe can also be
detected indirectly through its effect on the cosmic microwave background
(CMB) radiation (relic radiation from the Big Bang which permeates all
space). The radiation from the CMB would become polarized in the presence of
gravitational radiation. Detecting such polarized light is the mission of a
satellite based experiment (Planck) set to launch in 2009.

The gravitational radiation produced by either inflation or the mechanism
proposed by Jones-Smith, Krauss and Mathur would imprint itself on the CMB
and be detected as polarization. Until now it was widely believed that a
detection of polarized light from the CMB was a "smoking gun" for inflation
theory. But with the publication of their recent paper in Physical Review
Letters, Krauss and co-workers have raised the issue of whether that
polarized light can be unambiguously tied to inflation.

The mechanism proposed by Krauss and coworkers invokes a phenomenon called
"symmetry breaking" that is a central part of all theories of fundamental
particle physics, including the so-called standard model describing the
three non-gravitational forces known to exist. Here, 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.

Rich Blinne
Member ASA

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Received on Tue Jul 29 19:13:50 2008

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