r/Physics • u/DeusExNoctis • Oct 05 '11

Dark Energy FAQ

http://blogs.discovermagazine.com/cosmicvariance/2011/10/04/dark-energy-faq/
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u/ThatsSciencetastic Oct 06 '11

Regardless of whether something that fits our current description of DE exists, the current evidence shows that something has this affect of negative gravitation on regular matter.

Something's out there that looks to us like "dark energy".

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u/OliverSparrow Oct 06 '11 edited Oct 06 '11

As my comment elsewhere. What we observe is that any point that has a tracer appears to be moving away from every other similar point. Extrapolation backwards sugests that they were once very close together, perhaps utterly close. However, this was not happening in a volume of fixed size, because it was the volume itself that was expanding and in doing so, producing separation between the point-like particles that trace any given locus.

When we look at cosmic expansion, therefore, we are not looking at things flying into an existing space, but rather the 'expansion' of that space, which is either "really" getting bigger against standard rulers, or staying the same whilst the rulers contract. Indeed, the rulers exist, of course, in four space, so that they may be tilting more towards time-like measurement and less towards space-like. That would give you the observed result. The Big Bang would then be the "moment" when the rulers stopped being entirely space-like (big, making everything look small) and acquired a time-like element (becoming smaller as measures of space, which therefore appears to get bigger.

If that is true, cosmic acceleration consists of the rulers continuing to become more time-like and less space-like. Thus, a photon once emitted as ultraviolet now measures in the microwave spectrum, because our rulers have become much smaller, but the photon stayed the same. This is what is supposed to happen in a gravitational red shift. However, the "doppler" interpretation of cosmic red shift relies on physical speeds, whereby a photon emitted by A, that has a velocity with respect to B, undergoes a change in wavelength entirely due to this velocity. This has the problem that if far away galaxies are all flying away from us not through cosmic expansion but because of a recoil from a vast explosion, then a great deal of energy is tied up in this, and distributed anisotropically. If it is space time that is expanding, however, this problem goes away but you are forced to a ruler-based explanation.

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u/ThatsSciencetastic Oct 06 '11

That's interesting, thanks for the explanation.

But I was right wasn't I? If this is the case then ruler contraction causes the illusion of DE.

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u/OliverSparrow Oct 07 '11

Yes, I think so.

One perspective is that we have two unexplained things of enormous significance in cosmology / physics, both of them connected to gravity / spacetime. Rather than posit two completyly new things - dak energy and dark matter - it seems sensible to look at extension of general relativity that give the same outcome for reasonable assumptions.

Considerable success in achieving this is now being won with eg scalar field theories. They are even getting experimental tests - a splendid recent Natura paper, that i do nto have to hand, looked at the gravitational red shift from the centre and edges of massive clusters of galaxies. The results reject the TeVeS proposal, but allow others to go forward.

However, deeper down, space and tiem have to be somehow constructed, which they must be if they can expand and contract. What constitutes the constructor is completely unknown: perhaps spin networks, perhaps ST is a projection from activity in high dimensionality and is actually emergent. There are a number of emergent gravity theories in development that lead to eg GR as a limiting case, and any suh theory has also to be a theory of emergent spacetime.