| Topic: |
Science > Philosophy |
| User: |
"Sir Frederick" |
| Date: |
06 Nov 2004 05:03:41 PM |
| Object: |
On Dark Energy |
Cosmology: On Dark Energy
ScienceWeek http://scienceweek.com
The nature of the "dark energy" that is causing the apparent
accelerated expansion of the Universe is, without doubt, the
biggest mystery in physics and astronomy. Although it was
astrophysical observations of the acceleration that led to the
discovery of dark energy, there are precious few tests that can
be performed to work out what dark energy is.
COSMOLOGY: ON DARK ENERGY
The following points are made by Lawrence M. Krauss (Nature 2004
431:519):
1) The nature of the "dark energy" that is causing the apparent
accelerated expansion of the Universe is, without doubt, the
biggest mystery in physics and astronomy. Although it was
astrophysical observations of the acceleration that led to the
discovery of dark energy, there are precious few tests that can
be performed to work out what dark energy is -- whether it is
simply the rebirth of Einstein's cosmological constant, or
whether it might stem from something even weirder. All the
evidence so far is consistent with the existence of a
cosmological constant, which, in modern language, is understood
to be the quantum-mechanical energy associated with otherwise
empty space. Kunz et al(1) suggest, however, that by comparing
data on a range of astrophysical phenomena, it might be possible
to rule out a cosmological constant as the origin of dark energy.
2) Dark energy is perplexing. Physical theory currently has no
explanation of why the energy of empty space should be precisely
zero (quantum-mechanical effects combined with relativity in fact
predict quite the opposite). But it also gives no explanation of
why that energy should not instead be so huge that it would dwarf
all of the energy in anything else (making galaxy formation
impossible). Yet arguments based on a host of different
cosmological observations -- even before the direct observation
of the accelerated expansion -- implied that the energy in empty
space could not be more than three to four times greater than the
energy contained in the matter and radiation of the Universe. To
decide on what physics might be associated with dark energy, we
have to rely on experiments and observations. No laboratory
experiment we can imagine would be sensitive enough to do the
job, so we are left with astrophysical probes. Which is where
Kunz et al(1) come in.
3) Kunz et al(1) propose a three-way comparison of data: of the
expansion rate of the Universe as it changes with distance (from
measurements made using type-Ia supernovae, which originally led
to the discovery of dark energy(2,3)); with measurements(4) of
the temperature fluctuations in the cosmic microwave background
(the relic radiation of the Big Bang); and with measurements of
the clustering of galaxies on large scales. Studies of the cosmic
microwave background (CMB) have provided remarkably precise
constraints on most major cosmological parameters, and are in
some sense complementary to the limits derived using type-Ia
supernovae. To describe the different possibilities for dark
energy, an "equation-of-state" parameter, (w), is defined. This
is the ratio of the pressure to the energy of the material. For
the cosmological constant, (w) is exactly -1; any measured
difference from this value would signal the need for another
explanation. Data from the CMB, in combination with those from
supernovae, currently limit w to the range -1.2 < w < -0.8,
consistent with the value for a cosmological constant(4,5). For
comparison, (w) for matter is 0, and for radiation it is 1/3.
4) But Kunz et al(1) point out that allowance should be made for
a possible dynamical variation of (w) over time. The key new
ingredient they introduce is a comparison between the observed
clustering of matter on large scales across the Universe and the
predicted level of such clustering based on observations of the
fluctuations of the CMB. It turns out that, because of the way
that the dark energy comes to dominate the expansion of the
Universe, the CMB temperature fluctuations should change on the
largest angular scales (spanning more than about ten degrees
across the sky) in a way that is sensitive to the dark-energy
equation of state.
References (abridged):
1. Kunz, M., Corasaniti, P. -S., Parkinson, D. & Copeland, E. J.
Phys. Rev. D 70, 041301 (2004)
2. Schmidt, B. P. et al. Astrophys. J. 507, 46-63 (1998)
3. Perlmutter, S. J. et al. Astrophys. J. 517, 565-586 (1999)
4. Spergel, D. N. et al. Astrophys. J. Suppl. Ser. 148, 175-194
(2003)
5. Krauss, L. M. Astrophys. J. 604, 481-483 (2004)
Nature http://www.nature.com/nature
--------------------------------
Related Material:
THEORETICAL PHYSICS: ON THE SPACE-TIME VACUUM
The following points are made by R. B. Laughlin (Science 2004
303:1475):
1) In discussing cosmic matters it is impossible not to draw
analogies with science fiction from time to time, for the issues
are as large as those depicted in science fiction and equally
mysterious, despite being experimentally constrained.(1) Our
knowledge of the cosmos is still very primitive, and much of our
thinking about it correspondingly speculative, more along the
lines of what might plausibly have been than what is so.
Plausibility is an interesting concept in theoretical physics,
usually amounting to either a physical analogy with something
observed to occur elsewhere in nature or a mathematical
extrapolation of microscopic law. The latter, however, is
actually a shibboleth, for the things that matter are nearly
always collective organizational phenomena that cannot be
reliably predicted from microscopics. The shapes of galaxies and
the behavior of cosmic jets are simple cases in point, but the
observation also applies to the grandest issues of modern
cosmology: inflationary expansion and the hierarchical
consolidation of matter after the big bang (2-4). The absence of
predictive power is actually self-evident, because there would be
no point in measuring these things if one could calculate them.
As a practical matter, all plausibility arguments that count are
analogies.
2) It may seem shocking to speak of the vacuum of space-time as
an organizational phenomenon, but this is actually just a matter
of semantics. The idea behind the words is mainstream and fully
consistent with the facts. It has been known since the 1950s, and
routinely verified by accelerator experiments since then, that
empty space is a kind of matter quantum-mechanically similar to a
rock (5). The standard model of elementary particles is grounded
firmly on the idea of space as a phase. A multiplicity of such
phases and a complex sequence of transitions among them in the
early universe are corner-stones of modern particle cosmology.
The existence of such phases is implicated in the structure one
sees on intergalactic scales, and the heat released in the
transition between two of them is the ostensible power source of
inflation. Inflation itself is partly motivated by these phases,
because they make the observed uniformity of the universe
unnatural and something requiring explanation.
3) The semantic incongruity, however, like the sublimated worries
about modern life that give us science fiction nightmares, belies
something important -- unfinished business of the 1970s that has
been slowly and systematically tearing physics apart. Stripped of
their confusing mathematical descriptions, the phases of the
vacuum boil down to physical analogies with phases of ordinary
matter, natural phenomena observed to exhibit universality. That
means that their properties at long length and time scales, where
we normally do experiments, do not depend on microscopic details
at all, and thus do not constrain them when measured. A simple
example of emergent universality would be sound propagation in
fluids and solids, an effect perfectly well accounted for as the
motion of atoms, but also a generic property of the phases not
requiring atoms to make sense. Sound is an especially pertinent
example because it has a second identity at low temperatures as
an emergent elementary particle with properties identical to
those of particles of light. Insensitivity to microscopic detail
thus turns the concept of fundamental on its head, in that it
makes principles of self-organization the truly important thing,
rendering the quantum underpinnings of the Universe, whatever
they are, unknowable in the absence of experiments that reach
shorter scales and irrelevant to behavior we presently see.
Little wonder that physicists remain bitterly divided over full
acceptance of the vacuum as a phase.
References (abridged):
1. Akira, 124 min, directed by Katsuhiro Otomo (Kodansha Ltd.,
Japan, 1988)
2. S. Weinberg, The First Three Minutes: A Modern View of the
Origin of the Universe (Basic Books, New York, 1994)
3. M. Rees, New Perspectives in Astrophysical Cosmology
(Cambridge Univ. Press, Cambridge, 2000)
4. A. H. Guth, A. P. Lightman, The Inflationary Universe: The
Quest for a New Theory of Cosmic Origins (Perseus, New York,
1998)
5. M. E. Peskin, D. E Schroeder, An Introduction to Quantum Field
Theory (Westview, Boulder, CO, 1995)
Science http://www.sciencemag.org
--------------------------------
Related Material:
ON QUINTESSENCE AND THE EVOLUTION OF THE COSMOLOGICAL CONSTANT
The following points are made by P.J.E. Peebles (Nature 1999
398:25):
1) Contrary to expectations, the evidence is that the Universe is
expanding at approximately twice the velocity required to
overcome the gravitational pull of all the matter the Universe
contains. The implication of this is that in the past the greater
density of mass in the Universe gravitationally slowed the
expansion, while in the future the expansion rate will be close
to constant or perhaps increasing under the influence of a new
type of matter that some call "quintessence".
2) Quintessence began as Einstein's cosmological constant,
Lambda. It has negative gravitational mass: its gravity pushes
things apart.
3) Particle physicists later adopted Einstein's Lambda as a good
model for the gravitational effect of the active vacuum of
quantum physics, although the idea is at odds with the small
value of Lambda indicated by cosmology.
4) Theoretical cosmologists have noted that as the Universe
expands and cools, Lambda tends to decrease. As the Universe
cools, symmetries among forces are broken, particles acquire
masses, and these processes tend to release an analogue of latent
heat. The vacuum energy density accordingly decreases, and with
it the value of Lambda. Perhaps an enormous Lambda drove an early
rapid expansion that smoothed the primeval chaos to make the near
uniform Universe we see today, with a decrease in Lambda over
time to its current value. This is the cosmological inflation
concept.
5) The author suggests that the recent great advances in
detectors, telescopes, and observatories on the ground and in
space have given us a rough picture of what happened as our
Universe evolved from a dense, hot, and perhaps quite simple
early state to its present complexity. Observations in progress
are filling in the details, and that in turn is driving intense
debate on how the behavior of our Universe can be understood
within fundamental physics.
Nature http://www.nature.com/nature
--------------------------------
Notes by ScienceWeek:
Active vacuum of quantum physics: This refers to the idea that
the vacuum state in quantum mechanics has a zero-point energy
(minimum energy) which gives rise to vacuum fluctuations, so the
vacuum state does not mean a state of nothing, but is instead an
active state.
If a theory or process does not change when certain operations
are performed on it, the theory or process is said to possess a
symmetry with respect to those operations. For example, a circle
remains unchanged under rotation or reflection, and a circle
therefore has rotational and reflection symmetry. The term
"symmetry breaking" refers to the deviation from exact symmetry
exhibited by many physical systems, and in general, symmetry
breaking encompasses both "explicit" symmetry breaking and
"spontaneous" symmetry breaking. Explicit symmetry breaking is a
phenomenon in which a system is not quite, but almost, the same
for two configurations related by exact symmetry. Spontaneous
symmetry breaking refers to a situation in which the solution of
a set of physical equations fails to exhibit a symmetry possessed
by the equations themselves.
In general, the term "latent heat" refers to the quantity of heat
absorbed or released when a substance changes its physical phase
(e.g., solid to liquid) at constant temperature.
The inflationary model, first proposed by Alan Guth in 1980,
proposes that quantum fluctuations in the time period 10^(-35) to
10^(-32) seconds after time zero were quickly amplified into
large density variations during the "inflationary" 10^(50)
expansion of the Universe in that time frame.
ScienceWeek http://scienceweek.com
--
Best,
Frederick Martin McNeill
Poway, California, United States of America
mmcneill@fuzzysys.com
http://www.fuzzysys.com
http://members.cox.net/fmmcneill/
*************************
Phrase of the week :
For the real amazement, if you wish to be amazed, is this
process: You start out as a single cell derived from the coupling
of a sperm and an egg; this divides in two, then four, then
eight, and so on, and at a certain stage there emerges a single
cell which has as all its progeny the human brain. The mere
existence of such a cell should be one of the great astonishments
of the Earth. People ought to be walking around all day, all
through their waking hours calling to each other in endless
wonderment, talking of nothing except that cell. -- Lewis Thomas (1913-1993)
:-))))Snort!)
*************************
.
|
|
| User: "Joel Reicher" |
|
| Title: Re: On Dark Energy |
07 Nov 2004 02:57:58 AM |
|
|
Interesting. No philosophical content whatsoever, but interesting.
Cheers,
- Joel
.
|
|
|
| User: "Sir Frederick" |
|
| Title: Re: On Dark Energy |
07 Nov 2004 03:13:01 AM |
|
|
Joel Reicher wrote:
Interesting. No philosophical content whatsoever, but interesting.
Cheers,
- Joel
Worthy of philosophical comment. Where's yours?
.
|
|
|
| User: "Joel Reicher" |
|
| Title: Re: On Dark Energy |
07 Nov 2004 03:23:55 AM |
|
|
Sir Frederick <mmcneill@fuzzysys.com> writes:
Worthy of philosophical comment. Where's yours?
I'll have to take you word for it being worthy of philosophical
comment, because I can't think of any.
It's probably just me though.
Cheers,
- Joel
.
|
|
|
|
|
|
|
Related Articles |
|
|
