Dark Energy

[Advaithi]

I have heard Universe is expanding in accelerating speed because of Dark energy. What is Dark energy? If it is true when will Universe stop expanding? will it need any other force to slow down?

 

 

[Spyman]

It is called "Dark" because we don't yet know what it is but are able to observe its effects. Current observations indicates that it will never slow down. There is currently no known force able to bring the expansion to halt.

 

 

Dark energy

In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. Dark energy is the most accepted theory to explain recent observations and experiments that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for 73% of the total mass-energy of the universe.

http://en.wikipedia.org/wiki/Dark_energy

 

Future of an expanding universe

Observations suggest that the expansion of the universe will continue forever. If so, the universe will cool as it expands, eventually becoming too cold to sustain life. For this reason, this future scenario is popularly called the Big Freeze.

 

The future of an expanding universe is bleak. If a cosmological constant accelerates the expansion of the universe, the space between clusters of galaxies will grow at an increasing rate. Redshift will have stretched ancient, incoming photons (even gamma rays) to undetectably long wavelengths and low energies. Stars are expected to form normally for 1×10¹² to 1×10¹⁴ years, but eventually the supply of gas needed for star formation will be exhausted. Once the last star has exhausted its fuel, stars will cease to shine. According to theories that predict proton decay, the stellar remnants left behind would disappear, leaving behind only black holes which themselves eventually disappear as they emit Hawking radiation. Ultimately, if the universe reaches a state in which the temperature approaches a uniform value, no further work will be possible, resulting in a final heat death of the universe.

http://en.wikipedia.org/wiki/Future_of_an_expanding_universe

 

The Great Cosmic Battle

The expansion of the Universe itself provides an intensely dramatic example of the ubiquitous struggle between the force of gravity and entropy. As the Universe expands and becomes more spread out, gravity resists this trend and tries to pull the expanding Universe back together. The particular fate which our future holds depends on whether gravity wins or loses this cosmic battle, whose outcome depends on the total amount of mass and energy contained within the Universe. Current astronomical data strongly suggest that gravity has already lost this critical conflict and our fate will be determined by a continued and unending expansion.

http://www.astrosociety.org/pubs/mercury/0001/cosmic.html

Edited April 15, 2011 by Spyman

[Mr. X]

Has dark energy been proven to exist?

[Realitycheck]

Has dark energy been proven to exist?

Lots of technology has been designed and used just to study it and its effects, so it looks fairly irrefutable. While the phenomenon is undeniable, it simply isn't well understood, much like gravity, its opposing force. Considering that it cannot exactly be demonstrated right in front of you, like gravity can, makes it more difficult, but as observation has proven, its effect is undeniable. I highly doubt that the observation is erroneous.

[DrRocket]

Has dark energy been proven to exist?

 

No. At least in the sense of "to exist" in my personal lexicon.

 

There is a large and increasing body of evidence showing that space is expanding at an increasing rate, No one knows why. The cause has been dubbed "dark energy" so in that sense it exists. But no one understands what it is.

 

The energy of the vacuum shows up as a negative pressure term in the stress-energy tensor of general relativity in some analyses. That acts as a positive cosmological constant and would be an explanation for dark energy --except that attempts to perform the necessary calculation over-estimate the observed effect by about 120 orders of magnitude (by a factor of [math]10^{120}[/math]). That is a mistake so large as to be ludicrous.

 

Another approach is to simply insert a positive cosmological constant into the Einstein field equations, selected to match the observed expansion of space. That is what is done in the standard [math]\Lambda CDM[/math] model. But there is no explanation for the value of the cosmological constant "lambda".

 

 

So we know that space is expanding at an accelerated pace, but are clueless as to why. The "why" has been given a name -- dark energy. No one knows what dark energy is. If that counts as "exists", then yeah, it exists. I have somewhat higher standards for the term.

Edited April 26, 2011 by DrRocket

[Realitycheck]

The way I understood it is that expansion accelerated because once the universe reached a certain size, matter became diluted to a certain extent that gravity was no longer sufficient to slow down the effect of the cosmological constant.

Edited April 26, 2011 by Realitycheck

[Spyman]

But as DrRocket says, we don't know whats causing the effect of the cosmological constant.