[escepticos] The accelerated expansion of the universe (fwd)

["Victor R. Ruiz" <rvr@...>]

  Hola:

  Les paso esto, que está la mar de interesante, al respecto de esas
teorías hiperescépticas.

  Saludos,

Víctor R. Ruiz
rvr en idecnet.com

---------- Forwarded message ----------
Date: Sat, 06 Feb 1999 19:40:18 -0800
From: Ron Ebert <Ebert en citrus.ucr.edu>
To: ASTRO en lists.mindspring.com, skeptic en listproc.hcf.jhu.edu
Subject: [ASTRO] The accelerated expansion of the universe

In the recent past there has been discussion on both the Astro and Skeptic
list of supernova type Ia studies which imply the universe is in a state of
accelerated expansion, possibly due to a cosmological constant. There was a
recent documentary on BBC2 Horizon which has renewed interest in this
topic. Last Thursday we at the UC Riverside Physics Department had a
colloquium from one of the members of the two groups which have carried out
these studies, Alex Filippenko of UC Berkeley. I was impressed by the
presentation and the quality of the work but am still withholding judgement
on the reality of the accelerated expansion and its cause. I thought both
lists would like to know about some additional information I picked up and
some impressions I had about it.

In a type Ia supernova, a white dwarf accretes material from a nearby
companion star. In time, the amount of material exceeds the limit the white
dwarf can hold up and the white dwarf undergoes a supernova explosion as a
result. This type of supernova is often characterized by a lack of hydrogen
lines in their spectra, and when this happens the supernova is labeled as a
type Ia. There are characteristics of other spectral lines seen in type
Ia's, one of the most prominent being the spectral line of singly ionized
iron. The spectra of nearby supernovae match those of distant ones, and the
variation seen in either case is about 30 or 40 percent. Both teams think
they understand type Ia's well enough to compensate for these variations,
and so are able to use type Ia's as standard candles of distance. 

One result clearly seen from the spectra is that large redshifts are
consistent with the expansion of space, rather than with some competing
hypotheses such as "tired light." The expansion of space produces time
dilation and this spreads out the spectra in a measured way. There are a
handful of astrophysicists such as Arp and Burbidge who have disputed red
shifts as cosmological and have ascribed the red shifts to alternative
hypotheses such as tired light or the Wolf effect. The time dilation of the
type Ia spectra is a clear falsification of these alternatives.

The two studies also give one of the best estimates yet of the Hubble
constant. It's 65 +/- 6 km/s/Mpc (assuming no change in the pre-Hipparcos
zero point of the Cepheid distance scale). The measurements suggest a
current dynamical age of 14 billion years for the Universe, consistent with
the ages of globular star clusters.

The Hubble constant comes from Hubble's law for the expansion of the
universe: velocity = Hubble's constant x distance. This is a linear
relationship - when you plot redshift versus the relative intensity of
light for objects near and far, you're supposed to get a straight line.
(See for example, the diagram on page 51 of the January 1999 issue of
Scientific American.) But this is not what both teams see when they plot
the type Ia supernovae. The line is mostly straight, but it starts curving
up at high redshifts - those approaching Z = 1. If there are no unknown
observational or systemic errors in the observations, this has to mean the
universe is experiencing an accelerated expansion (See same diagram). Every
effort has been made to anticipate such errors. Some possibilities include
dimming of distant supernovae due to dust and the lower level of elements
heavier than helium in more distant and hence younger stars. The first
should show a skewing in the spectra, but this is not seen. The second
shouldn't be a factor since the spectra of distant type Ia's match nearby
ones. There are some other possibilities but I'll skip over these in the
interest of not overextending this post.

On the face of it, it would seem the accelerated expansion is real, but
Filippenko was properly cautious in interpreting his results. It's not a
good idea to rely on just one method of determining a parameter of the
universe, especially when alternatives are available. In a few years, the
Microwave Anisotropy Probe and Planck satellites will be launched. These
satellites will study the anisotropies in the cosmic microwave background
radiation to a very fine scale. The signature of the accelerated expansion
(along with many other characteristics about the universe) should be found
if the accelerated expansion is real. 

There is one other distinct signature to be found in further study of type
Ia's, but it doesn't look likely to beat the timing of results from MAP and
Planck. If the spectra of even more distant type Ia's are taken out to
redshifts of Z = 1.5 or so, the Hubble plot should start turning down if
the accelerated expansion is due to a cosmological constant. This would
show up because at such an early time in the history of the universe the CC
would not have had time to make its influence felt. On the other hand, if
the accelerated expansion is caused by quintessence, this will not be seen.
Such a unique signature would convince me for one that the CC is real, but
our present instrumentation is not good enough to take spectra of such
distant supernovae. We'll have to wait for the Hubble Telescope's successor
and infra-red observatories due to be launched in a few years.

The two current main ideas about the cause of the accelerated expansion, a
CC or quintessence, would have different effects. The CC would be caused by
a slight imbalance in the virtual particle sea, or vacuum energy, that
exists due to Heisenberg's Uncertainty Principle. Why this would be so
nobody knows, because if you add up the whole spectrum of virtual particles
spanning every possible wavelength, the calculated vacuum energy is roughly
120 orders of magnitude larger than the energy contained in all the matter
of the universe. There wouldn't be a universe with matter with a CC like
that - spacetime would have undergone continuous, nearly infinite fast
expansion from the first moments of its birth and light would have been
unable to travel from one point to another, let alone that particles would
be able to come together to combine into atomic nuclei and atoms. A CC is
truly constant. From the earliest moments of the universe its influence is
almost negligible, but as time goes on it becomes more and more dominant in
the expansion of the universe.

Quintessence wouldn't behave like this. The most popular idea about the
cause of quintessence is the decay of false vacuum objects like magnetic
monopoles or cosmic strings. When false vacuum decays to true vacuum, you
create more spacetime. If it is quintessence and this is the mechanism,
accelerated expansion should fall off as time goes on.

Filippenko said that he has talked to several superstring and inflationary
theorists about his results. Both camps are mighty unhappy with them. In
each case, accelerated expansion is inexplicable within the framework of
the theory. The theorists tell him he has to be wrong, but he replies that
this is an observational result and once confirmed they'll just have to
live with it. Filippenko also expressed annoyance over the BBC Horizon
production. He hasn't seen it but has talked to a good friend about it.
During production they interviewed Filippenko for about 2 hours, but it
seems they cut out all of his and other's cautions and caveats over the
results. How familiar that sounds to skeptics!


Ron Ebert
ron.ebert en ucr.edu
"""""""""""""""""""
Scientific theories are the BEST things we have to describe the world. If
something is "just a theory" it may not be perfect but it is just about as
good as it can be! - Bill Arnett, Astro List