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GarethMeredith
2015-Oct-06, 01:22 PM
Most people know, that the consensus today is the universe is accelerating. Now we think it isn't accelerating as fast.

In Hubble's law, it states that the further and further you get away, the faster you ''appear'' to accelerate!! This stands to reason then, that the most distant and perhaps earliest galaxies would appear to accelerate away. Why then, with a natural explanation of Hubble's law, did we [need] to adopt exotic theories of negative energy densities? Surely such a law would suffice on its own, if Hubble's law said to me that the most distant objects will appear to accelerate, then when we observed this I wouldn't have questioned the validity of such a statement. I do question bringing in superfluous subjects when they are not needed.

''Technically, the metric expansion of space is a feature of many solutions to the Einstein field equations of general relativity, and distance is measured using the Lorentz interval. This explains observations which indicate that galaxies that are more distant from us are receding faster than galaxies that are closer to us (Hubble's law).''

https://en.wikipedia.org/wiki/Metric_expansion_of_space#Hubble.27s_law

ngc3314
2015-Oct-06, 01:29 PM
In Hubble's law, it states that the further and further you get away, the faster you ''appear'' to accelerate

The relevant Hubble law (there are two more, much less well-known and not relevant here) says that there is a proportionality between a galaxy's distance and the redshift we measure for its light, one straightforward statement of which is that the relative velocity increases linearly with distance. No acceleration built into that; such may be inferred if we see evidence that the constant of proportionality is different in different redshift ranges, or ask what the time behavior of individual galaxies has to be to fit this pattern.

Jeff Root
2015-Oct-06, 03:53 PM
The way I understand Hubble's law is that at any given time,
there is a proportionality between distance and speed, and that
proportionality changes constantly -- though very slowly -- as
galaxies get farther apart, because the speeds of the galaxies
don't change. For example, at one particular time, a galaxy a
billion light-years away might have been moving away from us
at 1000 km/s, and at a later time that same galaxy would be
two billion light-years away and moving away at the same
speed of 1000 km/s, so the Hubble constant has decreased.

But it has been thought all along that the speeds of galaxies
*do* change: They constantly get slower because gravity is
pulling on them. A deceleration, an additional cause of a
decrease in the Hubble constant.

But on top of *that*, an acceleration was discovered in 1998
in which the speeds began increasing a few billion years ago.
An acceleration, causing an increase in the Hubble constant.

I think the OP wants these increases and decreases to cancel.

-- Jeff, in Minneapolis

mkline55
2015-Oct-06, 04:15 PM
But it has been thought all along that the speeds of galaxies
*do* change: They constantly get slower because gravity is
pulling on them. A deceleration, an additional cause of a
decrease in the Hubble constant.

That concept might work if the universe as a whole were finite in size. Is that the current belief?

Jeff Root
2015-Oct-06, 07:18 PM
Ken G has said that it works even if the Universe is infinite.

There is no consensus on whether the Universe is finite or
infinite.

I'll likely get rasberries from a couple of posters for repeating
this yet again, but my personal view is that the spatial extent
of the Universe that was involved in the Big Bang and is now
participating in the resulting cosmic expansion is necessarily
finite. If the Universe is infinite, then only a finite portion of
it was involved in the Big Bang.

-- Jeff, in Minneapolis

Reality Check
2015-Oct-06, 08:07 PM
Most people know, that the consensus today is the universe is accelerating. Now we think it isn't accelerating as fast.

In the words of Tonto: Who is "we", Kemo Sabe :D?
There are a couple of papers published in April 2015 that if found correct mean that type 1a supernovae have 2 populations, see Accelerating Universe: Not As Fast? (http://uanews.org/story/accelerating-universe-not-as-fast). The authors speculate that this will have an effect on the measurement of the rate of acceleration.
THE CHANGING FRACTIONS OF TYPE IA SUPERNOVA NUV–OPTICAL SUBCLASSES WITH REDSHIFT (http://iopscience.iop.org/article/10.1088/0004-637X/803/1/20/meta;jsessionid=165BE5B9FC2A01188D2638C84DC0CFF0.c 1)

...Not accounting for this effect should thus produce a distance bias that increases with redshift and could significantly bias measurements of cosmological parameters.
So "we" are waiting for confirmation of the result.

As other posters have noted, Hubble's law is that the further a galaxy is away from us, the faster we measure the galaxy to move (that is velocity, not acceleration). The observation of acceleration is Hubble's law being violated.

Reality Check
2015-Oct-06, 08:15 PM
I'll likely get rasberries from a couple of posters for repeating
this yet again, but my personal view is that the spatial extent
of the Universe that was involved in the Big Bang and is now
participating in the resulting cosmic expansion is necessarily
finite. If the Universe is infinite, then only a finite portion of
it was involved in the Big Bang.
Yes you will get a raspberry, Jeff Root :). It does not matter if the universe is finite or infinite - all of the universe is involved the Big Bang according to standard cosmology. The evidence is that the universe is flat and that implies infinite.
Whether the universe is finite or infinite then there is the possibility of "bubble" universes that are finite or infinite in extent.

Grey
2015-Oct-06, 08:16 PM
I'll likely get rasberries from a couple of posters for repeating
this yet again, but my personal view is that the spatial extent
of the Universe that was involved in the Big Bang and is now
participating in the resulting cosmic expansion is necessarily
finite. If the Universe is infinite, then only a finite portion of
it was involved in the Big Bang.Yes you will, because this is not the mainstream view, and yet you keep bringing it up in questions where it's not really relevant to the discussion. Best to have stopped after your second sentence and simply left your "personal view" out of it.

Grey
2015-Oct-06, 08:18 PM
You beat me to it by one minute, Reality Check. ;)

ShinAce
2015-Oct-06, 08:29 PM
Yay! I get to bust out my cosmology notes!

The so called 'Hubble parameter' is essentially the first derivative of the size of the universe with respect to time. You might as well call it the speed of the expanding universe.

What happens if you take the second derivative? Then you would obtain the so called 'deceleration parameter'. Then you go out and measure it with supernova and found out there is an acceleration. You might as well call it the acceleration of the expanding universe.
See https://en.wikipedia.org/wiki/Deceleration_parameter

You can now take the third derivative, and you might as well call it the jerk of the expanding universe.

The 'Hubble law' is just an approximation. A remarkably accurate one, but still nothing more than an approximation.

Comparing the Hubble law with universal acceleration only shows a misunderstanding of universal acceleration.

Grey
2015-Oct-06, 08:55 PM
...the jerk of the expanding universe.I've met some annoying people in my day, but I don't think there's been anyone bad enough to merit this title. ;)

ShinAce
2015-Oct-06, 09:11 PM
Lol!

Snap! Crackle! Pop! Kellogg's Universe!

John Mendenhall
2015-Oct-06, 09:47 PM
I've met some annoying people in my day, but I don't think there's been anyone bad enough to merit this title. ;)

buttercup has a very relevant thread, just opened today, here:

Cougar
2015-Oct-07, 02:30 AM
In Hubble's law, it states that the further and further you get away, the faster you ''appear'' to accelerate!!

Even if you drop off the bit about Hubble's law, which others have properly corrected, the rest of your statement is still incorrect. The history of the expansion rate is not very straightforward.

The velocity of the expansion must have been pretty high to begin with, I mean, even after inflation, say, a week later :). But all the mass was much closer together then, so the attracting gravity was stronger, and this slowed the expansion. Also, less space between objects means less dark energy between them, and its "accelerating effect" was insignificant early on.

Though it was slowing, the Universe was still expanding, of course. It was not slowed tremendouosly, so distances between remote objects (or the scale factor, AIUI) kept increasing, and the grip of gravity between objects weakened. Plus, more space means more dark energy. One model has the density of this "vacuum energy" stay constant even as the spatial volume increases. Its effect is obviously exceedingly small, but it adds up across large distances. If there's one thing the Universe has, it's large distances.

So there must have been a time when the expansion was expanding at a constant rate -- gravity had lost the battle, and the slowing of the expansion ceased. Initial measurements put this around 5 billion years ago1.

The expansion continued. The amount of space between objects in the Universe has always been pretty impressive, but after roughly 14 billion years of cosmic expansion, that is, at the present day, spatial distances are ridiculously large. There's a lot of space in the Universe, and that means a lot of dark energy, recent estimates of which have it accounting for 71% of the mass-energy of the visible Universe.

____________
1Goldsmith seems to have predicted Riess's subsequent finding as he wrote in The Runaway Universe [2000]:
"When astronomers succeed in observing supernovae with redshifts and distances much larger than those of the supernovae with redshifts between 0.4 and 0.7, the Hubble diagram for the universe actually reverts toward the original line describing a cosmos with no acceleration produced by a cosmological contant. This reversion occurs because as we look farther out in space, we look further back in time, to eras when the cosmological constant had produced a cumulative effect much smaller than at the present time or at times 'only' 4 to 7 billion years ago. We can effectively recapture the Hubble diagram for a universe without a cosmological constant by looking so far back in time that we observe epochs when the cosmological constant had produced negligible results."

Jeff Root
2015-Oct-07, 04:32 AM
I'll likely get rasberries from a couple of posters for repeating
this yet again, but my personal view is that the spatial extent
of the Universe that was involved in the Big Bang and is now
participating in the resulting cosmic expansion is necessarily
finite. If the Universe is infinite, then only a finite portion of
it was involved in the Big Bang.
Yes you will, because this is not the mainstream view,
and yet you keep bringing it up in questions where it's
not really relevant to the discussion.
Of course it is relevant. The expansion has to have a
mechanism, and accelerations and decelerations of the
expansion have to have mechanisms. Mechanisms that
might apply to an infinite universe are not all the same
as those that might apply to a finite universe.

-- Jeff, in Minneapolis

ShinAce
2015-Oct-07, 04:42 AM
Here's the universal history of what Cougar explained.

We can classify different times as 'eras' where a particular component of energy dominated the gravity scene. These components are: 1) radiation, 2) matter, 3) dark energy.

The radiation era is from 0-50,000 years. Believe it or not, the 'force' of gravity due to radiation(light) is actually stronger than the equivalent amount of matter(E=mc2).
The matter era is from about 50,000 to about 10 billion years. Gravity is still slowing down the expansion at this point.
The dark energy era is everything in the last 4 billion years and the foreseeable future. At this point, the expansion is accelerating.

So the magic point where space is expanding, but not accelerating it its expansion was about 4 billion years ago. That was the magic moment for astrophysics. From here on out, we'll eventually begin to lose sight of galaxies that Hubble can see today.

A Universe where the expansion is accelerating is bound to become a lonely place.

Noclevername
2015-Oct-07, 04:52 AM
I'll likely get rasberries from a couple of posters for repeating
this yet again, but my personal view is that the spatial extent
of the Universe that was involved in the Big Bang and is now
participating in the resulting cosmic expansion is necessarily
finite. If the Universe is infinite, then only a finite portion of
it was involved in the Big Bang.

Cougar
2015-Oct-07, 03:25 PM
Believe it or not, the 'force' of gravity due to radiation(light) is actually stronger than the equivalent amount of matter(E=mc2).

I'm not following this.... :confused-default:

Jeff Root
2015-Oct-07, 04:23 PM
Everything was so hot that there were far more photons than
fermions, and the photons were ridiculously high-frequency.
So there was more energy in the photons than in the matter,
despite matter's huge advantage over light in energy density.
And that means the majority of the gravity present was due
to the light.

-- Jeff, in Minneapolis

ShinAce
2015-Oct-07, 06:08 PM
In one word....pressure. When we think of gravity, we think of the energy content. But you must also include the pressure.

Dark energy has negative pressure. Matter has approximately 0 pressure. Radiation has pressure which is needed to solve the time evolution of the universe.

Take a static universe made of matter. Allow some of the matter to annihilate into photons. What is the Universe's fate then? Collapse!!!