The Universe Might Be Expanding a Lot Slower Than We Thought • Hacker News Board

Re: The Universe Might Be Expanding a Lot Slower Than We Thought

#11

Post by antognini » Sun, Apr 12, 2015, 4:37 PM UTC

GravityWell wrote: post #9

antognini wrote: post #7

Earlier quoted context omitted.

I'm a graduate student in astronomy and I think it's extremely unlikely that dark energy or dark matter will go away. The concern of an "error cascade" was legitimate when these ideas were first proposed and many astronomers were skeptical of both dark matter and dark energy for long while. But over time there have been a number of independent lines of evidence pointing to the existence of both dark matter and dark e…

Care to share some examples of dark energy evidence? If the evidence is a measurement of expansion, then it doesn't necessarily support the existence of dark energy.

One of the other major pieces of evidence for the existence of dark energy is the observation of baryon acoustic oscillations. The basic idea is that as the universe was forming, there were concentrations of dark matter. Standing sound waves of photons and baryons (ordinary matter) formed around these concentrations. After decoupling, the photons streamed away, but the baryons remained and eventually formed galaxies. This results in a characteristic pattern seen on the sky, where there are clusters of galaxies observed, and then rings of galaxies around them.

It is possible to calculate the physical size of these rings from first principles. Since you observe how big these rings appear on the sky, you can figure out how far away they are, and from this determine the expansion history of the universe.

Re: The Universe Might Be Expanding a Lot Slower Than We Thought

#12

Post by pdkl95 » Sun, Apr 12, 2015, 4:46 PM UTC

GravityWell wrote: post #9

antognini wrote: post #7

Earlier quoted context omitted.

I'm a graduate student in astronomy and I think it's extremely unlikely that dark energy or dark matter will go away. The concern of an "error cascade" was legitimate when these ideas were first proposed and many astronomers were skeptical of both dark matter and dark energy for long while. But over time there have been a number of independent lines of evidence pointing to the existence of both dark matter and dark e…

Care to share some examples of dark energy evidence? If the evidence is a measurement of expansion, then it doesn't necessarily support the existence of dark energy.

https://medium.com/starts-with-a-bang/five-reasons-we-think-...

While I'm not an astronomer, I think observations such as the the Bullet Cluster and the different position s for where the mass seems to be - including big changes in morphology, not just location - clearly indicates that something dark matter/energy-like must exist. The precise nature of this DM/DE is an a separate question.

Re: The Universe Might Be Expanding a Lot Slower Than We Thought

#13

Post by [deleted] » Sun, Apr 12, 2015, 4:46 PM UTC

GravityWell wrote: post #9

antognini wrote: post #7

Earlier quoted context omitted.

I'm a graduate student in astronomy and I think it's extremely unlikely that dark energy or dark matter will go away. The concern of an "error cascade" was legitimate when these ideas were first proposed and many astronomers were skeptical of both dark matter and dark energy for long while. But over time there have been a number of independent lines of evidence pointing to the existence of both dark matter and dark e…

Care to share some examples of dark energy evidence? If the evidence is a measurement of expansion, then it doesn't necessarily support the existence of dark energy.

[deleted]

Re: The Universe Might Be Expanding a Lot Slower Than We Thought

#14

Post by spdustin » Sun, Apr 12, 2015, 4:53 PM UTC

stephenwinter2 wrote: post #10

ehvatum wrote: post #8

Earlier quoted context omitted.

Space itself expanding, ie inflation, doesn't require the universe to have a center of mass.

Exactly. Thanks for your affirmation. .. And space itself expanding doesn't require 'dark energy' and has nothing to do with momentum, acceleration etc.

Can you expand on why you believe that the expansion of space itself requires no energy?

Re: The Universe Might Be Expanding a Lot Slower Than We Thought

#15

Post by stephenwinter2 » Sun, Apr 12, 2015, 5:14 PM UTC

stephenwinter2 wrote: post #10

ehvatum wrote: post #8

Earlier quoted context omitted.

Space itself expanding, ie inflation, doesn't require the universe to have a center of mass.

Exactly. Thanks for your affirmation. .. And space itself expanding doesn't require 'dark energy' and has nothing to do with momentum, acceleration etc.

> spdustin That's the uncertain part of the question. It's the same when you ask Does time require a force that drives it forward? The answer is We don't know yet. -- The certain part of the question is: scaling of space (in cosmic scale) can't be accelerated by 'dark energy' nor slowed down by gravity. It's not about mass, energy, momentum etc. - I really described it in my paper (see post above).

Re: The Universe Might Be Expanding a Lot Slower Than We Thought

#16

Post by vorg » Sun, Apr 12, 2015, 8:57 PM UTC

antognini wrote: post #11

GravityWell wrote: post #9

Earlier quoted context omitted.

Care to share some examples of dark energy evidence? If the evidence is a measurement of expansion, then it doesn't necessarily support the existence of dark energy.

One of the other major pieces of evidence for the existence of dark energy is the observation of baryon acoustic oscillations. The basic idea is that as the universe was forming, there were concentrations of dark matter. Standing sound waves of photons and baryons (ordinary matter) formed around these concentrations. After decoupling, the photons streamed away, but the baryons remained and eventually formed galaxies.…

> as the universe was forming, there were concentrations of dark matter. Standing sound waves of photons and baryons (ordinary matter) formed around these concentrations

You make it sound like "dark matter" is normal matter and the "ordinary matter" is the abnormal stuff, since ordinary matter formed afterwards and has more complex interactions in that it supplements gravitation with electromagnetism and what not. And of course there's far more dark matter than ordinary.

Re: The Universe Might Be Expanding a Lot Slower Than We Thought

#17

Post by spdustin » Mon, Apr 13, 2015, 4:27 PM UTC

stephenwinter2 wrote: post #15

stephenwinter2 wrote: post #10

Earlier quoted context omitted.

Exactly. Thanks for your affirmation. .. And space itself expanding doesn't require 'dark energy' and has nothing to do with momentum, acceleration etc.

> spdustin That's the uncertain part of the question. It's the same when you ask Does time require a force that drives it forward? The answer is We don't know yet. -- The certain part of the question is: scaling of space (in cosmic scale) can't be accelerated by 'dark energy' nor slowed down by gravity. It's not about mass, energy, momentum etc. - I really described it in my paper (see post above).

I had understood that time is a side effect of entropy.