Dark Energy: Exploring Negative Pressure's Role in Universe Expansion

[RJ Emery]

TL;DR Summary

Accelerating Expansion of the Universe

In an article published in Scientific American, authors Adam Riess and Mario Livio wrote:

Scientists have a number of hypotheses for what might be driving the acceleration of the universe. The leading candidate arises from the nature of empty space. In quantum physics a vacuum is not “nothing”—rather it is teeming with pairs of “virtual” particles and antiparticles that spontaneously appear and annihilate one another within a tiny fraction of a second. As strange as it may sound, this sea of ephemeral particle pairs carries energy, and energy, just like mass, can produce gravity. Unlike mass, however, energy can create either an attractive or a repulsive gravity, depending on whether its pressure is positive or negative. The vacuum energy in empty space, according to theory, should have a negative pressure and thus may be the source of the repulsive gravity driving the accelerated expansion of the universe. — Scientific American, March, 2016, page 40,

Why negative pressure? I would think the pressure would be positive, thus making gravity repulsive.

 

[phinds]

Why negative pressure? I would think the pressure would be positive, thus making gravity repulsive.

Gravity is positivity attractive, thus a negative would make it repulsive. Hm ... this is beginning to sound like a girl I dated in college

 

[berkeman]

Gravity is positivity attractive, thus a negative would make it repulsive. Hm ... this is beginning to sound like a girl I dated in college

TMI...

 

[PeterDonis]

Summary:: Accelerating Expansion of the Universe

In an article published in Scientific American

Please provide a link.

 

[PeterDonis]

Summary:: Accelerating Expansion of the Universe

Why negative pressure?

See this article:

http://www.astro.ucla.edu/~wright/cosmo_constant.html

 

[PeterDonis]

I would think the pressure would be positive, thus making gravity repulsive.

Positive pressure causes attractive gravity, not repulsive gravity. Roughly speaking, if you're looking at a perfect fluid, its "gravity" is described by $\rho + 3 p$, where $\rho$ is the energy density and $p$ is the pressure; the more positive this quantity is, the more attractive gravity is. So positive pressure adds to attractive gravity. (Dark energy has equation of state $p = - \rho$, so $\rho + 3 p = - 2 \rho$, which is negative since $\rho$ is positive. That's why dark energy causes "repulsive gravity".)

 

[RJ Emery]

Please provide a link.

A link to what? The original article online? It is behind a paywall, and unless you have a paid subscription to SA (I do) you won't be able to access it. Furthermore, I quoted the relevant paragraph. That's all there is to it. If you wish to read the full article, visit your local library.

 

[PeterDonis]

A link to what? The original article online?

Yes.

It is behind a paywall, and unless you have a paid subscription to SA (I do) you won't be able to access it.

And some readers may have such a subscription. In any case, when you are asked for a reference, you need to give it. That is PF policy, which you signed up to when you became a member.

I quoted the relevant paragraph.

Doesn't matter. We still need a link.

If you wish to read the full article, visit your local library.

If you wish to continue posting here, please abide by the rules.

 

[RJ Emery]

Yes.And some readers may have such a subscription. In any case, when you are asked for a reference, you need to give it. That is PF policy, which you signed up to when you became a member.Doesn't matter. We still need a link.If you wish to continue posting here, please abide by the rules.

If you bothered to read the original post, I gave the publication, issue date, and page number. The article is not available online in the public domain. You will not be getting a link because there is no link.

 

[TeethWhitener]

https://www.scientificamerican.com/article/dark-energy-no-answers-but-more-questions/
Try harder, @RJ Emery

 

[PeterDonis]

If you bothered to read the original post, I gave the publication, issue date, and page number. The article is not available online in the public domain. You will not be getting a link because there is no link.

This is not an acceptable response. Thread closed.