Science > Physics > What's the difference between dark matter and dark energy?
| Topic: |
Science > Physics |
| User: |
"Sam Wormley" |
| Date: |
21 Sep 2005 11:27:46 AM |
| Object: |
What's the difference between dark matter and dark energy? |
What's the difference between dark matter and dark energy?
From "Ask the Astronomer"
http://curious.astro.cornell.edu/question.php?number=634
Question:
I've been reading many clever answers here about dark matter and dark
energy that called my attention to this question. Since Einstein's
theory relates matter and energy as different states of the same thing,
is it valid to think about dark matter and dark energy in the same way?
Are they two states of the same dark "thing"? Are they interchangeable?
Answer:
The short answer to your question is that we don't know if dark matter
and dark energy are manifestations of the same dark "thing". We know
they both must exist to explain certain phenomena, but we still know
very little about their make up so we cannot assume they are linked.
For now, we think of them as separate, and we believe the cosmos to be
composed of roughly 0.03% heavy elements (anything other than hydrogen
and helium), 0.3% neutrinos, 0.5% stars, 4% free hydrogen and helium,
25% dark matter, and 70% dark energy. Here is how we define them
separately:
Dark matter must exist to account for the gravity that holds galaxies
together. If the only matter in the universe was matter we could
directly detect, galaxies would not have had enough matter to have ever
formed. The galaxies we observe today would fly apart because they
wouldn't have enough matter to create a strong enough gravitational
force to hold themselves together. Dark matter is also responsible for
amplifying small fluctuations in the Cosmic Microwave Background back
in the early universe to create the large scale structure we observe in
the universe today.
Dark energy, which also goes by the names of the cosmological constant
or quintessence, must exist due to the rate of expansion we observe for
our universe. Not only is the universe expanding, but this expansion is
also accelerating so the unknown 'anti-gravity' force at work is termed
'dark energy'.
Some researchers are searching for an explanation that encompasses both
dark matter and dark energy. One example of such a theory uses a form
of energy called a scalar field (it is a field because it has
magnitude, energy and pressure, but it is scalar so it has no
direction). Things would certainly be easier if we didn't need to have
separate theories to explain dark matter and dark energy. However,
other researchers look at dark matter and dark energy as two separate
problems. For example, many string theories use super symmetric
particles to explain dark matter and make no connection to dark energy
at all.
See: http://curious.astro.cornell.edu/question.php?number=634
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| User: "OsherD" |
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| Title: Re: What's the difference between dark matter and dark energy? |
21 Sep 2005 01:18:06 PM |
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From Osher Doctorow
Sam Wormley typed:
What's the difference between dark >matter and dark energy?
and he discussed some different viewpoints about the largely unresolved
issue. Such questions are very useful and hopefully will prevent
proliferation of anomalies and paradoxes which old time researchers
arguably allowed to continue for often decades because they paid
insufficient attention to "second and third opinions" and to
reexamining foundations of their own theories and comformist
tendencies.
Osher
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| User: "Ben Rudiak-Gould" |
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| Title: Re: What's the difference between dark matter and dark energy? |
21 Sep 2005 02:44:01 PM |
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Sam Wormley wrote:
From "Ask the Astronomer"
http://curious.astro.cornell.edu/question.php?number=634
Question:
[...] Since Einstein's
theory relates matter and energy as different states of the same thing,
is it valid to think about dark matter and dark energy in the same way?
[...]
Answer:
The short answer to your question is that we don't know if dark matter
and dark energy are manifestations of the same dark "thing". [...]
Not a very good answer. Despite going on at great length, it never really
addresses the original question.
The answer to the original question is that "dark matter" and "dark energy"
are arbitrary labels chosen by astronomers to refer to particular things,
not two-word phrases that can be analyzed by the meanings of the individual
words.
"Dark matter" is stuff that behaves like ordinary matter -- in particular,
the total amount of it is conserved, so the average density of it goes down
as the universe expands. "Dark energy" has a constant density everywhere and
everywhen, so there's more and more of it as the universe expands. That's
the difference between them, and there's no way to figure that out from the
difference between the words "matter" and "energy".
-- Ben
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| User: "Autymn D. C." |
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| Title: Re: What's the difference between dark matter and dark energy? |
22 Sep 2005 11:01:55 PM |
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any tests that dark matter has normal inertia?
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