I was listening to a talk about the size, age, and expansion of the universe recently and the topic of dark energy and accelerating expansion came up.
So the question is this: If dark energy is accelerating the expansion of the universe then that should apply to earth - withing our local group of galaxies. So why don't we feel that acceleration?
It's possible that the acceleration is to small to detect. Even trying to find out how fast we are moving relative to the CMBR isn't easy. This NASA site:
"On Sun, 11 Nov 2012 12:59:10 -0500, in article <k7op1f$qp...@dont-email.me>,
Mark Buchanan stated..."
>I was listening to a talk about the size, age, and expansion of the >universe recently and the topic of dark energy and accelerating >expansion came up.
>So the question is this: If dark energy is accelerating the expansion of >the universe then that should apply to earth - withing our local group >of galaxies. So why don't we feel that acceleration?
I am not an astronomer or physicist, so take this as a request for
enlightenment rather than a response:
My understanding is that within the Local Group of galaxies (including
the Milky Way Galaxy, the Magellanic Clouds, the Andromeda Galaxy and
so on), cosmological effects are not significant - gravity is more
important. Dark Energy has an effect only on a scale of billions of
light years, rather than mere (!) millions.
>It's possible that the acceleration is to small to detect. Even trying >to find out how fast we are moving relative to the CMBR isn't easy. This >NASA site:
Mark Buchanan <marklynn.bucha...@gmail.com> wrote:
>I was listening to a talk about the size, age, and expansion of the >universe recently and the topic of dark energy and accelerating >expansion came up.
>So the question is this: If dark energy is accelerating the expansion of >the universe then that should apply to earth - withing our local group >of galaxies. So why don't we feel that acceleration?
>It's possible that the acceleration is to small to detect. Even trying >to find out how fast we are moving relative to the CMBR isn't easy. This >NASA site:
>http://apod.nasa.gov/apod/ap030209.html >says it's 600 k/s but this Wiki article
>http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation >says it's 371 k/s towards Leo - no reference given though.
>Is there someone who knows in the NG?
The expansion doesn't work quite that way. It is the fabric
of space that is expanding.
To use the old balloon analogy, imagine a two-dimensional universe
on the surface of a balloon. Mark stars with bits of sticky paper.
Now make the balloon bigger. The sticky bits of paper get further
away from each other. But the sticky bits of paper don't expand.
Gravity holds them together.
Same in our three-D universe. Gravity keeps everything that
was close together together.
> "On Sun, 11 Nov 2012 12:59:10 -0500, in article<k7op1f$qp...@dont-email.me>,
> Mark Buchanan stated..."
>> I was listening to a talk about the size, age, and expansion of the
>> universe recently and the topic of dark energy and accelerating
>> expansion came up.
>> So the question is this: If dark energy is accelerating the expansion of
>> the universe then that should apply to earth - withing our local group
>> of galaxies. So why don't we feel that acceleration?
> I am not an astronomer or physicist, so take this as a request for
> enlightenment rather than a response:
I'm not either.
> My understanding is that within the Local Group of galaxies (including
> the Milky Way Galaxy, the Magellanic Clouds, the Andromeda Galaxy and
> so on), cosmological effects are not significant - gravity is more
> important. Dark Energy has an effect only on a scale of billions of
> light years, rather than mere (!) millions.
Understood but supposedly anything happening on the scale of billions of light years away from us is also happening to us - since we are no more at the center of the universe than anything else.
>> It's possible that the acceleration is to small to detect. Even trying
>> to find out how fast we are moving relative to the CMBR isn't easy. This
>> NASA site:
> Mark Buchanan<marklynn.bucha...@gmail.com> wrote:
>> I was listening to a talk about the size, age, and expansion of the
>> universe recently and the topic of dark energy and accelerating
>> expansion came up.
>> So the question is this: If dark energy is accelerating the expansion of
>> the universe then that should apply to earth - withing our local group
>> of galaxies. So why don't we feel that acceleration?
>> It's possible that the acceleration is to small to detect. Even trying
>> to find out how fast we are moving relative to the CMBR isn't easy. This
>> NASA site:
>> says it's 371 k/s towards Leo - no reference given though.
>> Is there someone who knows in the NG?
> The expansion doesn't work quite that way. It is the fabric
> of space that is expanding.
> To use the old balloon analogy, imagine a two-dimensional universe
> on the surface of a balloon. Mark stars with bits of sticky paper.
> Now make the balloon bigger. The sticky bits of paper get further
> away from each other. But the sticky bits of paper don't expand.
> Gravity holds them together.
> Same in our three-D universe. Gravity keeps everything that
> was close together together.
I do understand the local dynamics of our own little cluster of galaxies - at least in principle. What I don't get is this: distant galaxies are moving away from each other at speeds relative to their distances from each other but all speeds should be slowing down, not speeding up. So if everything is speeding up away from each other shouldn't we as well experience that acceleration?
The person giving the talk is a physicist himself. When I asked the question he didn't know the answer.
This paper discusses this in more details but seems to be older (no date given):
<marklynn.bucha...@gmail.com> wrote:
>On 11/11/2012 1:36 PM, Paul J Gans wrote:
>> Mark Buchanan<marklynn.bucha...@gmail.com> wrote:
>>> I was listening to a talk about the size, age, and expansion of the
>>> universe recently and the topic of dark energy and accelerating
>>> expansion came up.
>>> So the question is this: If dark energy is accelerating the expansion of
>>> the universe then that should apply to earth - withing our local group
>>> of galaxies. So why don't we feel that acceleration?
>>> It's possible that the acceleration is to small to detect. Even trying
>>> to find out how fast we are moving relative to the CMBR isn't easy. This
>>> NASA site:
>>> says it's 371 k/s towards Leo - no reference given though.
>>> Is there someone who knows in the NG?
>> The expansion doesn't work quite that way. It is the fabric
>> of space that is expanding.
>> To use the old balloon analogy, imagine a two-dimensional universe
>> on the surface of a balloon. Mark stars with bits of sticky paper.
>> Now make the balloon bigger. The sticky bits of paper get further
>> away from each other. But the sticky bits of paper don't expand.
>> Gravity holds them together.
>> Same in our three-D universe. Gravity keeps everything that
>> was close together together.
>I do understand the local dynamics of our own little cluster of galaxies >- at least in principle. What I don't get is this: distant galaxies are >moving away from each other at speeds relative to their distances from >each other but all speeds should be slowing down, not speeding up. So if >everything is speeding up away from each other shouldn't we as well >experience that acceleration?
You don't say why you think all speeds should be slowing down.
Gravity?
ISTM Paul's explanation was pretty good, with a small minor nit: the
fabric of space also permeates the sticky papers, but there just isn't
enough space within them to overcome their internal gravity, as Paul
said.
OTOH and AIUI as the Universe expands, so the amount of space between
any two points in the Universe increases. IOW the more the Universe
expands, the faster it expands.
Mark Buchanan wrote:
> I was listening to a talk about the size, age, and expansion of the
> universe recently and the topic of dark energy and accelerating
> expansion came up.
> So the question is this: If dark energy is accelerating the expansion
> of the universe then that should apply to earth - withing our local
> group of galaxies. So why don't we feel that acceleration?
> It's possible that the acceleration is to small to detect. Even trying
> to find out how fast we are moving relative to the CMBR isn't easy.
> This NASA site:
> says it's 371 k/s towards Leo - no reference given though.
> Is there someone who knows in the NG?
> Mark
The same wikipedia article says that the velocity of the Earth (I think they mean the Sun, because the Earth would produce an orbital variation through the year) relative to the CMBR is 371 km/s, but that the Local Group of Galaxies (presumably its centre of mass) is moving at 627 km/sec relative to the CMBR. The latter figure is found from the Sun's orbital speed and direction around the galactic centre and the best estimate of the Galaxy's velocity relative to the Local Group centre of mass.
Mark Buchanan <marklynn.bucha...@gmail.com> wrote:
> I was listening to a talk about the size, age, and expansion of the > universe recently and the topic of dark energy and accelerating > expansion came up.
> So the question is this: If dark energy is accelerating the expansion of
> the universe then that should apply to earth - withing our local group
> of galaxies. So why don't we feel that acceleration?
> It's possible that the acceleration is to small to detect. Even trying
> to find out how fast we are moving relative to the CMBR isn't easy.
Nonsense. The speed is accurately known.
Even the yearly variations due to the earth orbiting the sun
are easily measurable.
> says it's 371 k/s towards Leo - no reference given though.
Everything is relative.
The speed depends on which of the known local speeds
you add or subtract.
The 600 km/s is the speed of the whole local group (of galaxies)
wrt the cmb.
IIRC the 371 km/s is the speed of the solar system.
> Mark Buchanan wrote:
>> I was listening to a talk about the size, age, and expansion of the
>> universe recently and the topic of dark energy and accelerating
>> expansion came up.
>> So the question is this: If dark energy is accelerating the expansion
>> of the universe then that should apply to earth - withing our local
>> group of galaxies. So why don't we feel that acceleration?
>> It's possible that the acceleration is to small to detect. Even trying
>> to find out how fast we are moving relative to the CMBR isn't easy.
>> This NASA site:
>> says it's 371 k/s towards Leo - no reference given though.
>> Is there someone who knows in the NG?
>> Mark
> The same wikipedia article says that the velocity of the Earth (I think they
> mean the Sun, because the Earth would produce an orbital variation through
> the year) relative to the CMBR is 371 km/s, but that the Local Group of
> Galaxies (presumably its centre of mass) is moving at 627 km/sec relative to
> the CMBR. The latter figure is found from the Sun's orbital speed and
> direction around the galactic centre and the best estimate of the Galaxy's
> velocity relative to the Local Group centre of mass.
OK, so the local group is moving 627 k/s - my initial question remains unanswered. Is the 627 k/s changing over time?
My understanding is that no one knows why we are moving 627 k/s towards Leo in the first place.
> On Sun, 11 Nov 2012 14:26:04 -0500, Mark Buchanan
> <marklynn.bucha...@gmail.com> wrote:
>> On 11/11/2012 1:36 PM, Paul J Gans wrote:
>>> Mark Buchanan<marklynn.bucha...@gmail.com> wrote:
>>>> I was listening to a talk about the size, age, and expansion of the
>>>> universe recently and the topic of dark energy and accelerating
>>>> expansion came up.
>>>> So the question is this: If dark energy is accelerating the expansion of
>>>> the universe then that should apply to earth - withing our local group
>>>> of galaxies. So why don't we feel that acceleration?
>>>> It's possible that the acceleration is to small to detect. Even trying
>>>> to find out how fast we are moving relative to the CMBR isn't easy. This
>>>> NASA site:
>>>> says it's 371 k/s towards Leo - no reference given though.
>>>> Is there someone who knows in the NG?
>>> The expansion doesn't work quite that way. It is the fabric
>>> of space that is expanding.
>>> To use the old balloon analogy, imagine a two-dimensional universe
>>> on the surface of a balloon. Mark stars with bits of sticky paper.
>>> Now make the balloon bigger. The sticky bits of paper get further
>>> away from each other. But the sticky bits of paper don't expand.
>>> Gravity holds them together.
>>> Same in our three-D universe. Gravity keeps everything that
>>> was close together together.
>> I do understand the local dynamics of our own little cluster of galaxies
>> - at least in principle. What I don't get is this: distant galaxies are
>> moving away from each other at speeds relative to their distances from
>> each other but all speeds should be slowing down, not speeding up. So if
>> everything is speeding up away from each other shouldn't we as well
>> experience that acceleration?
> You don't say why you think all speeds should be slowing down.
> Gravity?
Of course it's gravity - and it's not just what I think - the fact that the expansion of the universe is accelerating was a surprise to cosmologists.
> ISTM Paul's explanation was pretty good, with a small minor nit: the
> fabric of space also permeates the sticky papers, but there just isn't
> enough space within them to overcome their internal gravity, as Paul
> said.
I understand the balloon thing well enough - I don't 'get' the concept 'fabric of space'.
> OTOH and AIUI as the Universe expands, so the amount of space between
> any two points in the Universe increases. IOW the more the Universe
> expands, the faster it expands.
<marklynn.bucha...@gmail.com> wrote:
>On 11/11/2012 3:46 PM, jillery wrote:
>> On Sun, 11 Nov 2012 14:26:04 -0500, Mark Buchanan
>> <marklynn.bucha...@gmail.com> wrote:
>>> On 11/11/2012 1:36 PM, Paul J Gans wrote:
>>>> Mark Buchanan<marklynn.bucha...@gmail.com> wrote:
>>>>> I was listening to a talk about the size, age, and expansion of the
>>>>> universe recently and the topic of dark energy and accelerating
>>>>> expansion came up.
>>>>> So the question is this: If dark energy is accelerating the expansion of
>>>>> the universe then that should apply to earth - withing our local group
>>>>> of galaxies. So why don't we feel that acceleration?
>>>>> It's possible that the acceleration is to small to detect. Even trying
>>>>> to find out how fast we are moving relative to the CMBR isn't easy. This
>>>>> NASA site:
>>>>> says it's 371 k/s towards Leo - no reference given though.
>>>>> Is there someone who knows in the NG?
>>>> The expansion doesn't work quite that way. It is the fabric
>>>> of space that is expanding.
>>>> To use the old balloon analogy, imagine a two-dimensional universe
>>>> on the surface of a balloon. Mark stars with bits of sticky paper.
>>>> Now make the balloon bigger. The sticky bits of paper get further
>>>> away from each other. But the sticky bits of paper don't expand.
>>>> Gravity holds them together.
>>>> Same in our three-D universe. Gravity keeps everything that
>>>> was close together together.
>>> I do understand the local dynamics of our own little cluster of galaxies
>>> - at least in principle. What I don't get is this: distant galaxies are
>>> moving away from each other at speeds relative to their distances from
>>> each other but all speeds should be slowing down, not speeding up. So if
>>> everything is speeding up away from each other shouldn't we as well
>>> experience that acceleration?
>> You don't say why you think all speeds should be slowing down.
>> Gravity?
>Of course it's gravity - and it's not just what I think - the fact that >the expansion of the universe is accelerating was a surprise to >cosmologists.
Why of course gravity? Astronomers have known for some time there
isn't enough normal matter to slow down the Universe's expansion.
Astronomers have also known for some time that something is wrong
about their understanding of gravity at the galactic scale. They
calculated that all the matter they can see within galaxies isn't
enough to explain the motions of stars within them. And they see
gravitational lensing that can't be explained by the matter they can
see. So they invented dark matter, some mysterious substance that
doesn't emit or absorb photons but still has gravitational effects.
The acceleration of expansion is an entirely different thing. Before
they looked at those distant supernovae, there was no reason to
suppose that the rate was accelerating. Now astronomy has another
great mystery, that it has an effect but no idea what is the cause.
>> ISTM Paul's explanation was pretty good, with a small minor nit: the
>> fabric of space also permeates the sticky papers, but there just isn't
>> enough space within them to overcome their internal gravity, as Paul
>> said.
>I understand the balloon thing well enough - I don't 'get' the concept >'fabric of space'.
It is a metaphor, and that might be what is confusing you. Perhaps
you could elaborate more fully on what you don't get, taking into
account what has already been said.
>> OTOH and AIUI as the Universe expands, so the amount of space between
>> any two points in the Universe increases. IOW the more the Universe
>> expands, the faster it expands.
>Without 'dark energy' this wouldn't be true.
Again, no. AIUI the 'effect' of accelerated expansion is more-or-less
established. It's not clear at all what is the actual 'cause'. The
phrase "dark energy" is purely a placeholder label.
Mark Buchanan wrote:
> On 11/11/2012 5:31 PM, Mike Dworetsky wrote:
>> Mark Buchanan wrote:
>>> I was listening to a talk about the size, age, and expansion of the
>>> universe recently and the topic of dark energy and accelerating
>>> expansion came up.
>>> So the question is this: If dark energy is accelerating the
>>> expansion of the universe then that should apply to earth - withing
>>> our local group of galaxies. So why don't we feel that acceleration?
>>> It's possible that the acceleration is to small to detect. Even
>>> trying to find out how fast we are moving relative to the CMBR
>>> isn't easy. This NASA site:
>>> says it's 371 k/s towards Leo - no reference given though.
>>> Is there someone who knows in the NG?
>>> Mark
>> The same wikipedia article says that the velocity of the Earth (I
>> think they mean the Sun, because the Earth would produce an orbital
>> variation through the year) relative to the CMBR is 371 km/s, but
>> that the Local Group of Galaxies (presumably its centre of mass) is
>> moving at 627 km/sec relative to the CMBR. The latter figure is
>> found from the Sun's orbital speed and direction around the galactic
>> centre and the best estimate of the Galaxy's velocity relative to
>> the Local Group centre of mass.
> OK, so the local group is moving 627 k/s - my initial question remains
> unanswered. Is the 627 k/s changing over time?
> My understanding is that no one knows why we are moving 627 k/s
> towards Leo in the first place.
> Mark
Random (or orbital) motions of clusters of galaxies relative to one another can reach speeds of order 500-1000 km/sec. This can be seen on plots of redshift vs distance as scatter. Further scatter comes from uncertain measurements of distance.
So the answer to "why" is "random motions". There is absolutely no reason why any particular collection of galaxies such as the Local Group should be at rest with respect to the CMBR.
The 627 km/sec is not changing with time except possibly on a timescale of billions of years, due to the Local Group orbiting around larger clusters such as Virgo, or acceleration towards the Great Attractor (a very large giant cluster or supercluster towards which there seems to be a gravitational flow of galaxy clusters in our vicinity).
In article <k7op1f$qp...@dont-email.me>,
Mark Buchanan <marklynn.bucha...@gmail.com> wrote:
> I was listening to a talk about the size, age, and expansion of the > universe recently and the topic of dark energy and accelerating > expansion came up.
> So the question is this: If dark energy is accelerating the expansion of > the universe then that should apply to earth - withing our local group > of galaxies. So why don't we feel that acceleration?
> It's possible that the acceleration is to small to detect.
That's right -- too small by a very large margin. There is
still some debate about the exact amount -- it's a complicated
question in general relativity, and a solution of the relevant
equations requires a number of approximations. But the answer is certainly tiny: for the Solar System, by one reasonable calculation, it's 44 orders of magnitude smaller than the acceleration of the Earth due to the Sun's gravity (see http://arxiv.org/abs/astro-ph/9803097). At the galaxy cluster scale, the expansion induced by cosmological expansion is larger, but still about seven orders of magnitude smaller than the local gravitational accelerations.
Price has argued (see http://arxiv.org/abs/gr-qc/0508052) that for an exponentially expanding Universe, there's actually no
local effect, but this seems to be a special case.
If the rate of acceleration of the Universe is itself increasing
-- a so-called "big rip" scenario -- it will *eventually* be
important at local scales, but we'll have a very long wait.
Steven Carlip <car...@physics.ucdavis.edu> wrote:
> In article <k7op1f$qp...@dont-email.me>,
> Mark Buchanan <marklynn.bucha...@gmail.com> wrote:
> > I was listening to a talk about the size, age, and expansion of the
> > universe recently and the topic of dark energy and accelerating > > expansion came up.
> > So the question is this: If dark energy is accelerating the expansion of
> > the universe then that should apply to earth - withing our local group
> > of galaxies. So why don't we feel that acceleration?
> > It's possible that the acceleration is to small to detect.
> That's right -- too small by a very large margin. There is
> still some debate about the exact amount -- it's a complicated
> question in general relativity, and a solution of the relevant
> equations requires a number of approximations. But the answer > is certainly tiny: for the Solar System, by one reasonable > calculation, it's 44 orders of magnitude smaller than the > acceleration of the Earth due to the Sun's gravity (see > http://arxiv.org/abs/astro-ph/9803097). At the galaxy cluster > scale, the expansion induced by cosmological expansion is larger, > but still about seven orders of magnitude smaller than the local > gravitational accelerations.
> Price has argued (see http://arxiv.org/abs/gr-qc/0508052) that > for an exponentially expanding Universe, there's actually no
> local effect, but this seems to be a special case.
> If the rate of acceleration of the Universe is itself increasing
> -- a so-called "big rip" scenario -- it will *eventually* be
> important at local scales, but we'll have a very long wait.
I hate waiting. Can we rip it now?
-- John S. Wilkins, Associate, Philosophy, University of Sydney
http://evolvingthoughts.net But al be that he was a philosophre,
Yet hadde he but litel gold in cofre
> In article <k7op1f$qp...@dont-email.me>,
> Mark Buchanan <marklynn.bucha...@gmail.com> wrote:
>> I was listening to a talk about the size, age, and expansion of the
>> universe recently and the topic of dark energy and accelerating
>> expansion came up.
>> So the question is this: If dark energy is accelerating the expansion of
>> the universe then that should apply to earth - withing our local group
>> of galaxies. So why don't we feel that acceleration?
>> It's possible that the acceleration is to small to detect.
> That's right -- too small by a very large margin. There is
> still some debate about the exact amount -- it's a complicated
> question in general relativity, and a solution of the relevant
> equations requires a number of approximations. But the answer
> is certainly tiny: for the Solar System, by one reasonable
> calculation, it's 44 orders of magnitude smaller than the
> acceleration of the Earth due to the Sun's gravity (see
> http://arxiv.org/abs/astro-ph/9803097). At the galaxy cluster
> scale, the expansion induced by cosmological expansion is larger,
> but still about seven orders of magnitude smaller than the local
> gravitational accelerations.
> Price has argued (see http://arxiv.org/abs/gr-qc/0508052) that
> for an exponentially expanding Universe, there's actually no
> local effect, but this seems to be a special case.
> If the rate of acceleration of the Universe is itself increasing
> -- a so-called "big rip" scenario -- it will *eventually* be
> important at local scales, but we'll have a very long wait.
> "On Sun, 11 Nov 2012 12:59:10 -0500, in article <k7op1f$qp...@dont-email.me>,
> Mark Buchanan stated..."
>> I was listening to a talk about the size, age, and expansion of the
>> universe recently and the topic of dark energy and accelerating
>> expansion came up.
>> So the question is this: If dark energy is accelerating the expansion of
>> the universe then that should apply to earth - withing our local group
>> of galaxies. So why don't we feel that acceleration?
> I am not an astronomer or physicist, so take this as a request for
> enlightenment rather than a response:
> My understanding is that within the Local Group of galaxies (including
> the Milky Way Galaxy, the Magellanic Clouds, the Andromeda Galaxy and
> so on), cosmological effects are not significant - gravity is more
> important. Dark Energy has an effect only on a scale of billions of
> light years, rather than mere (!) millions.
In fact, the Milky Way Galaxy and the Andromeda Galaxy are moving closer. May collide in another 4 billion years or so.
> > "On Sun, 11 Nov 2012 12:59:10 -0500, in article <k7op1f$qp...@dont-email.me>,
> > Mark Buchanan stated..."
> >> I was listening to a talk about the size, age, and expansion of the
> >> universe recently and the topic of dark energy and accelerating
> >> expansion came up.
> >> So the question is this: If dark energy is accelerating the expansion of
> >> the universe then that should apply to earth - withing our local group
> >> of galaxies. So why don't we feel that acceleration?
> > I am not an astronomer or physicist, so take this as a request for
> > enlightenment rather than a response:
> > My understanding is that within the Local Group of galaxies (including
> > the Milky Way Galaxy, the Magellanic Clouds, the Andromeda Galaxy and
> > so on), cosmological effects are not significant - gravity is more
> > important. Dark Energy has an effect only on a scale of billions of
> > light years, rather than mere (!) millions.
> In fact, the Milky Way Galaxy and the Andromeda Galaxy are moving
> closer. May collide in another 4 billion years or so.
<car...@physics.ucdavis.edu> wrote:
>In article <k7op1f$qp...@dont-email.me>,
> Mark Buchanan <marklynn.bucha...@gmail.com> wrote:
>> I was listening to a talk about the size, age, and expansion of the >> universe recently and the topic of dark energy and accelerating >> expansion came up.
>> So the question is this: If dark energy is accelerating the expansion of >> the universe then that should apply to earth - withing our local group >> of galaxies. So why don't we feel that acceleration?
>> It's possible that the acceleration is to small to detect.
>That's right -- too small by a very large margin. There is
>still some debate about the exact amount -- it's a complicated
>question in general relativity, and a solution of the relevant
>equations requires a number of approximations. But the answer >is certainly tiny: for the Solar System, by one reasonable >calculation, it's 44 orders of magnitude smaller than the >acceleration of the Earth due to the Sun's gravity (see >http://arxiv.org/abs/astro-ph/9803097). At the galaxy cluster >scale, the expansion induced by cosmological expansion is larger, >but still about seven orders of magnitude smaller than the local >gravitational accelerations.
>Price has argued (see http://arxiv.org/abs/gr-qc/0508052) that >for an exponentially expanding Universe, there's actually no
>local effect, but this seems to be a special case.
>If the rate of acceleration of the Universe is itself increasing
>-- a so-called "big rip" scenario -- it will *eventually* be
>important at local scales, but we'll have a very long wait.
>Steve Carlip
Thank you Steve for posting this. As you may recall, I have
tremendous respect for the time you take to provide quality replies.
I am not an expert in these things, so it's no surprise to me that
your answer leaves me with some questions. I hope I can phrase them
well enough that you can understand them.
First, I remain unclear how an accelerating expansion of space applies
to Earth. I don't expect any object, Earth included, to be at rest
relative to the CMB, but I missed how that relative motion is a
consequence of the accelerating expansion.
Second, I remain unclear how the Big Rip becomes important at local
scales. Am I correct in my understanding that the accelerating
expansion of space is a consequence of the increasing amount of space?
If so, then if gravitationally-bound objects remain so even as space
expands around them, why wouldn't gravitationally-bound objects remain
so even as the expansion accelerates?
Thank you in advance for helping me clear up my confusion.
> On Nov 12, 11:57 pm, "Steven L." <sdlit...@earthlink.net> wrote:
>> On 11/11/2012 1:17 PM, TomS wrote:
>>> "On Sun, 11 Nov 2012 12:59:10 -0500, in article <k7op1f$qp...@dont-email.me>,
>>> Mark Buchanan stated..."
>>>> I was listening to a talk about the size, age, and expansion of the
>>>> universe recently and the topic of dark energy and accelerating
>>>> expansion came up.
>>>> So the question is this: If dark energy is accelerating the expansion of
>>>> the universe then that should apply to earth - withing our local group
>>>> of galaxies. So why don't we feel that acceleration?
>>> I am not an astronomer or physicist, so take this as a request for
>>> enlightenment rather than a response:
>>> My understanding is that within the Local Group of galaxies (including
>>> the Milky Way Galaxy, the Magellanic Clouds, the Andromeda Galaxy and
>>> so on), cosmological effects are not significant - gravity is more
>>> important. Dark Energy has an effect only on a scale of billions of
>>> light years, rather than mere (!) millions.
>> In fact, the Milky Way Galaxy and the Andromeda Galaxy are moving
>> closer. May collide in another 4 billion years or so.
> On Sun, 11 Nov 2012 21:24:57 -0500, Mark Buchanan
> <marklynn.bucha...@gmail.com> wrote:
>> On 11/11/2012 3:46 PM, jillery wrote:
>>> On Sun, 11 Nov 2012 14:26:04 -0500, Mark Buchanan
>>> <marklynn.bucha...@gmail.com> wrote:
>>>> On 11/11/2012 1:36 PM, Paul J Gans wrote:
>>>>> Mark Buchanan<marklynn.bucha...@gmail.com> wrote:
>>>>>> I was listening to a talk about the size, age, and expansion of the
>>>>>> universe recently and the topic of dark energy and accelerating
>>>>>> expansion came up.
>>>>>> So the question is this: If dark energy is accelerating the expansion of
>>>>>> the universe then that should apply to earth - withing our local group
>>>>>> of galaxies. So why don't we feel that acceleration?
>>>>>> It's possible that the acceleration is to small to detect. Even trying
>>>>>> to find out how fast we are moving relative to the CMBR isn't easy. This
>>>>>> NASA site:
>>>>>> says it's 371 k/s towards Leo - no reference given though.
>>>>>> Is there someone who knows in the NG?
>>>>> The expansion doesn't work quite that way. It is the fabric
>>>>> of space that is expanding.
>>>>> To use the old balloon analogy, imagine a two-dimensional universe
>>>>> on the surface of a balloon. Mark stars with bits of sticky paper.
>>>>> Now make the balloon bigger. The sticky bits of paper get further
>>>>> away from each other. But the sticky bits of paper don't expand.
>>>>> Gravity holds them together.
>>>>> Same in our three-D universe. Gravity keeps everything that
>>>>> was close together together.
>>>> I do understand the local dynamics of our own little cluster of galaxies
>>>> - at least in principle. What I don't get is this: distant galaxies are
>>>> moving away from each other at speeds relative to their distances from
>>>> each other but all speeds should be slowing down, not speeding up. So if
>>>> everything is speeding up away from each other shouldn't we as well
>>>> experience that acceleration?
>>> You don't say why you think all speeds should be slowing down.
>>> Gravity?
>> Of course it's gravity - and it's not just what I think - the fact that
>> the expansion of the universe is accelerating was a surprise to
>> cosmologists.
> Why of course gravity? Astronomers have known for some time there
> isn't enough normal matter to slow down the Universe's expansion.
> Astronomers have also known for some time that something is wrong
> about their understanding of gravity at the galactic scale. They
> calculated that all the matter they can see within galaxies isn't
> enough to explain the motions of stars within them. And they see
> gravitational lensing that can't be explained by the matter they can
> see. So they invented dark matter, some mysterious substance that
> doesn't emit or absorb photons but still has gravitational effects.
> The acceleration of expansion is an entirely different thing. Before
> they looked at those distant supernovae, there was no reason to
> suppose that the rate was accelerating. Now astronomy has another
> great mystery, that it has an effect but no idea what is the cause.
>>> ISTM Paul's explanation was pretty good, with a small minor nit: the
>>> fabric of space also permeates the sticky papers, but there just isn't
>>> enough space within them to overcome their internal gravity, as Paul
>>> said.
>> I understand the balloon thing well enough - I don't 'get' the concept
>> 'fabric of space'.
> It is a metaphor, and that might be what is confusing you. Perhaps
> you could elaborate more fully on what you don't get, taking into
> account what has already been said.
>>> OTOH and AIUI as the Universe expands, so the amount of space between
>>> any two points in the Universe increases. IOW the more the Universe
>>> expands, the faster it expands.
>> Without 'dark energy' this wouldn't be true.
> Again, no. AIUI the 'effect' of accelerated expansion is more-or-less
> established. It's not clear at all what is the actual 'cause'. The
> phrase "dark energy" is purely a placeholder label.
<marklynn.bucha...@gmail.com> wrote:
>On 11/11/2012 10:37 PM, jillery wrote:
>> On Sun, 11 Nov 2012 21:24:57 -0500, Mark Buchanan
>> <marklynn.bucha...@gmail.com> wrote:
>>> On 11/11/2012 3:46 PM, jillery wrote:
>>>> On Sun, 11 Nov 2012 14:26:04 -0500, Mark Buchanan
>>>> <marklynn.bucha...@gmail.com> wrote:
>>>>> On 11/11/2012 1:36 PM, Paul J Gans wrote:
>>>>>> Mark Buchanan<marklynn.bucha...@gmail.com> wrote:
>>>>>>> I was listening to a talk about the size, age, and expansion of the
>>>>>>> universe recently and the topic of dark energy and accelerating
>>>>>>> expansion came up.
>>>>>>> So the question is this: If dark energy is accelerating the expansion of
>>>>>>> the universe then that should apply to earth - withing our local group
>>>>>>> of galaxies. So why don't we feel that acceleration?
>>>>>>> It's possible that the acceleration is to small to detect. Even trying
>>>>>>> to find out how fast we are moving relative to the CMBR isn't easy. This
>>>>>>> NASA site:
>>>>>>> says it's 371 k/s towards Leo - no reference given though.
>>>>>>> Is there someone who knows in the NG?
>>>>>> The expansion doesn't work quite that way. It is the fabric
>>>>>> of space that is expanding.
>>>>>> To use the old balloon analogy, imagine a two-dimensional universe
>>>>>> on the surface of a balloon. Mark stars with bits of sticky paper.
>>>>>> Now make the balloon bigger. The sticky bits of paper get further
>>>>>> away from each other. But the sticky bits of paper don't expand.
>>>>>> Gravity holds them together.
>>>>>> Same in our three-D universe. Gravity keeps everything that
>>>>>> was close together together.
>>>>> I do understand the local dynamics of our own little cluster of galaxies
>>>>> - at least in principle. What I don't get is this: distant galaxies are
>>>>> moving away from each other at speeds relative to their distances from
>>>>> each other but all speeds should be slowing down, not speeding up. So if
>>>>> everything is speeding up away from each other shouldn't we as well
>>>>> experience that acceleration?
>>>> You don't say why you think all speeds should be slowing down.
>>>> Gravity?
>>> Of course it's gravity - and it's not just what I think - the fact that
>>> the expansion of the universe is accelerating was a surprise to
>>> cosmologists.
>> Why of course gravity? Astronomers have known for some time there
>> isn't enough normal matter to slow down the Universe's expansion.
>> Astronomers have also known for some time that something is wrong
>> about their understanding of gravity at the galactic scale. They
>> calculated that all the matter they can see within galaxies isn't
>> enough to explain the motions of stars within them. And they see
>> gravitational lensing that can't be explained by the matter they can
>> see. So they invented dark matter, some mysterious substance that
>> doesn't emit or absorb photons but still has gravitational effects.
>> The acceleration of expansion is an entirely different thing. Before
>> they looked at those distant supernovae, there was no reason to
>> suppose that the rate was accelerating. Now astronomy has another
>> great mystery, that it has an effect but no idea what is the cause.
>>>> ISTM Paul's explanation was pretty good, with a small minor nit: the
>>>> fabric of space also permeates the sticky papers, but there just isn't
>>>> enough space within them to overcome their internal gravity, as Paul
>>>> said.
>>> I understand the balloon thing well enough - I don't 'get' the concept
>>> 'fabric of space'.
>> It is a metaphor, and that might be what is confusing you. Perhaps
>> you could elaborate more fully on what you don't get, taking into
>> account what has already been said.
>>>> OTOH and AIUI as the Universe expands, so the amount of space between
>>>> any two points in the Universe increases. IOW the more the Universe
>>>> expands, the faster it expands.
>>> Without 'dark energy' this wouldn't be true.
>> Again, no. AIUI the 'effect' of accelerated expansion is more-or-less
>> established. It's not clear at all what is the actual 'cause'. The
>> phrase "dark energy" is purely a placeholder label.
> Steven Carlip <car...@physics.ucdavis.edu> wrote:
> > In article <k7op1f$qp...@dont-email.me>,
> > Mark Buchanan <marklynn.bucha...@gmail.com> wrote:
> > > I was listening to a talk about the size, age, and expansion of the
> > > universe recently and the topic of dark energy and accelerating > > > expansion came up.
> > > So the question is this: If dark energy is accelerating the expansion of
> > > the universe then that should apply to earth - withing our local group
> > > of galaxies. So why don't we feel that acceleration?
> > > It's possible that the acceleration is to small to detect.
> > That's right -- too small by a very large margin. There is
> > still some debate about the exact amount -- it's a complicated
> > question in general relativity, and a solution of the relevant
> > equations requires a number of approximations. But the answer > > is certainly tiny: for the Solar System, by one reasonable > > calculation, it's 44 orders of magnitude smaller than the > > acceleration of the Earth due to the Sun's gravity (see > > http://arxiv.org/abs/astro-ph/9803097). At the galaxy cluster > > scale, the expansion induced by cosmological expansion is larger, > > but still about seven orders of magnitude smaller than the local > > gravitational accelerations.
> > Price has argued (see http://arxiv.org/abs/gr-qc/0508052) that > > for an exponentially expanding Universe, there's actually no
> > local effect, but this seems to be a special case.
> > If the rate of acceleration of the Universe is itself increasing
> > -- a so-called "big rip" scenario -- it will *eventually* be
> > important at local scales, but we'll have a very long wait.
> I hate waiting. Can we rip it now?
Don't worry, you'll be dead long before you have to worry about local effects of the rip tide.
> I was listening to a talk about the size, age, and expansion of the > universe recently and the topic of dark energy and accelerating > expansion came up.
> So the question is this: If dark energy is accelerating the expansion of > the universe then that should apply to earth - withing our local group > of galaxies. So why don't we feel that acceleration?
> It's possible that the acceleration is to small to detect. Even trying > to find out how fast we are moving relative to the CMBR isn't easy. This > NASA site:
> I was listening to a talk about the size, age, and expansion of the
> universe recently and the topic of dark energy and accelerating
> expansion came up.
> So the question is this: If dark energy is accelerating the expansion of
> the universe then that should apply to earth - withing our local group
> of galaxies. So why don't we feel that acceleration?
