Discovering Dark Energy and Dark Matter in a Single Galaxy

[wolram]

Would there be any way to discover dark energy, dark matter, if the only
observable body was the galaxy you lived in?

 

[marcus]

Intriguing question. I can't see how DE could be detected.

I think DM was deduced just from Milkyway data several decades ago. People realized (perhaps over 30 years ago) that Milky was whirling too fast to hold together, unless there were some "missing matter" that had not been detected, contributing gravity.

DM is just an hypothesis to address the "missing matter" puzzle. I think those ideas would come up even if there were just one observable galaxy.

But the DE question is a beaut. There might be some way that other people here can think of, but I can't see any.

 

[Wallace]

You wouldn't be able discover dark energy the way we have. On the other hand we don't know what dark energy is (if it even exists) so we may in the future be able to do laboratory experiments with dark energy, in this case your hypothetical island galaxy inhabitants could do the same. It's shear speculation that this will ever be possible though, so the short answer I think is no.

On DM I think you could have suspicions about its existence as suggested by marcus. Remember as well though that galaxies are only one way we see the effects of dark matter. We also see the need for it to explain the dynamics of clusters of galaxies as well as its influence on the global properties of the Universe. I'm not sure if the 'answer' to the rotation problem would be so clear if you only saw the view of a single galaxy from the inside.

 

[wolram]

Thankyou Marcus, Wallace.
I was hoping there was some means to detect DE by looking for some effect to the quantum world, but it seems it is to weak to show up there, and needs a vast volume to be able to do any work.
But when i think about it every particle must (feel) dark energy, now i am confused.

 

[Wallace]

But when i think about it every particle must (feel) dark energy, now i am confused.

Only through gravity though, not necessarily through any of the other forces of nature (weak, strong, electro-mag). There are plenty of particles that see some but not all of the fundamental forces. The only one we think all of them feel is gravity (though only particle physicists still think of gravity as a force, in GR there is no force of gravity)

 

[marcus]

Wolram's question is considered in detail in the beautiful paper by Larry Krauss and Bob Scherrer

http://arxiv.org/abs/0704.0221
The Return of a Static Universe and the End of Cosmology
Lawrence M. Krauss (1,2), Robert J. Scherrer (2) ((1) Case Western Reserve University, (2) Vanderbilt University)
submitted to 2007 Gravity Research Foundation Essay Competition

"We demonstrate that as we extrapolate the current LambdaCDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our "island universe" will be fundamentally incapable of determining the true nature of the universe, including the existence of the highly dominant vacuum energy, the existence of the CMB, and the primordial origin of light elements. With these pillars of the modern Big Bang gone, this epoch will mark the end of cosmology and the return of a static universe. In this sense, the coordinate system appropriate for future observers will perhaps fittingly resemble the static coordinate system in which the de Sitter universe was first presented."

IMHO this is an unusually easy to read essay
which is beautiful and enlightening, I would urge anyone who has not read it to do so.

I think Wolram may have read it when it came out earlier this month and if so it could have prompted this thread.

Krauss and Scherrer have entered the essay in the annual gravity essay competition for 2007 and my private hunch is that they will at least place if not win first prize.
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EDIT I WILL REPLY HERE INSTEAD OF MAKING ANOTHER POST. well sad melancholy visions can be beautiful and educational but they don't necessarily come true. Krauss paper is like a poem in the elegaic tradition.
it is exquisitely reasoned and crafted. but it only comes true if things are unalterably exactly as he assumes.
I want to admire it enthusiastically but not go so far as let it depress me.

REPLYING FURTHER: IMO if one takes diffeo-invariance seriously it is precisely the events which are NOT gauge, and humanity's existence and accomplishments consiste of events, so I think post #10 needs a wink to signify ironical humor (lest someone get the wrong idea)

 

[Wallace]

I read that paper as well. It's a neat (if somewhat depressing) idea. I guess we are lucky to be living now and not 100 billion years in the future so that we can discover so many of the secrets of out universe.

Then again perhaps the anthropic principle means that we couldn't have existed 100 billion years from now so it's not luck that is to be thanked!

 

[ukmicky]

Island universe

Unless they are asuming to much and the universe is far more than we realize. Couldnt we be part of an island universe ,where we are resident in one of trillions of expanding spots/universes which will all merge to become one super universe in billions or trillions of years time.

Just thinking :)

 

[Wallace]

Well yes, the paper extrapolate current theory to describe our future universe. Of course this extrapolation will change if our theories do.

As to your suggestion, yes it could be correct. But so could a bazillion other possibilities. We don't however, have any reason such as any evidence or observations that suggest this is the case.

 

[MeJennifer]

Then again perhaps the anthropic principle means that we couldn't have existed 100 billion years from now so it's not luck that is to be thanked!

True, but if we take diffeomorphism invariance seriously, human existence is just Gauge.

 

[ukmicky]

when you think about it if the universe or should i say the cosmos goes on for ever it makes sense to believe that there could be a much big picture,and that our universe as we know it isn't the be all and end all.

 

[Wallace]

Okay, I'm going to be flippant here, but your statement in logically equivalent to "When you think about it Zombies would be much more likely to wear roller skates than anti-gravity jump packs". This point is that we can make all sorts of speculations but without any evidence to check them with there is little point. Just don't get me started on string theory

 

[ukmicky]

Okay, I'm going to be flippant here, but your statement in logically equivalent to "When you think about it Zombies would be much more likely to wear roller skates than anti-gravity jump packs".

No Completly different .
We used to look to the skys and think we were the be all and end all , then we looked further and found that our solar system was part of a galaxy and that our galaxy was simply just one of billions of galaxies and we are now trying to look even further and one day someone with vision will look even further to see what's there and maybe you never know we may find another piece to the puzzle.

I'm do nothing different to what the island universe authors are doing after all they are theorising without knowing the big picture. They are asuming that there is nothing coming our way from beyond our deep field scans of the universe witout looking beyond and you accept what they say .

They even say and i quote "so that observers in our "island universe" " That says to me that they don't discount the idea that our universe is not on its own. and your not being flippant in regards to them and their thoughts :-)

 

[Wallace]

No, what these authors did is crank the handle of existing theory (based on evidence) to give a description of the future. What you are doing is making up a future based entirely on speculation that there is no evidence for.

Of course in the future we may uncover new evidence that will update our theories. However to take your description of the history of astronomy, before we found, from improved telescope observations, that the Universe was much bigger than our own galaxy, suggesting that this was the case was also sheer speculation. I'm sure some people must have had those thoughts, but there must have been many other thoughts as well. Until we have the evidence the speculation is pointless.

It won't take someone with some kind of heroic boldfaced vision to see 'further' than we currently do, what it will take is a lot of hard work, money and time spent on bigger and better telescopes. There are plenty of these 'visionary' projects already in the works that will no doubt discover all kinds of wondrous new things about the Universe that will in all likelihood be far more amazing that we could ever have imagined.

The Universe is beautiful and better telescopes reveal more and more of this beauty. It dosn't take idle un-verifiable speculation to make it any more so!

 

[ukmicky]

"Until we have the evidence the speculation is pointless."

"Pointless "

How can you say that afterall there must be countless discoveries made due to someone speculating and its not like its a wild idea afterall all the galaxies which we can see through our telescopes is evidence enough for speculation of other universes to be justified.

 

[Wallace]

Sure, to be more clear, I'm referring to speculating about things we cannot measure or observe. Predicting what a certain suggestion about the Universe would lead to in terms of something that we can measure or hope to measure in a reasonable time frame is what science is about.

Speculating about things we cannot measure or hope to measure in the future is pointless since you can never say that the speculation is right or wrong. We can observe as far as our past light cone permits, and we do this when we look at the CMB. Anything 'further' than this is by definition, not within the 'observable universe' and hence we can never test any theory that relies on observations beyond this!

 

[marcus]

Unless they are asuming to much and the universe is far more than we realize. Couldnt we be part of an island universe ,where we are resident in one of trillions of expanding spots/universes which will all merge to become one super universe in billions or trillions of years time.
...

I'm do nothing different to what the island universe authors are doing after all they are theorising without knowing the big picture. They are asuming that there is nothing coming our way from beyond our deep field scans of the universe witout looking beyond and you accept what they say .

Where in the paper does it say they assume that? I am sure that the authors do NOT assume there is nothing "coming our way" out beyond the cosmic event horizon. There must be tons of stuff moving in our direction, at a whole range of possible speeds. So I have to suppose that you didn't understand something, Mickey, or that you made up about their assuming there wasnt.

But if you can find where they say "nothing out there coming our way" then please show me the quote in their paper.

It is more fun if you understand the standard LambdaCDM model first before you start making up alternatives.

If you would please first understand the usual LCDM consensus model, and what Krauss Scherrer conclude from it, and then start disagreeing, you would come across as a more imaginative person.

Krauss Scherrer take for granted a huge multitude of galaxies our beyond what we can observe, I am sure. Astronomers normally do. they certainly do not explicitly state the contrary!

So when you say the same thing---and speak of trillions of galaxies out beyond our ken---it comes across as kind of old. It doesn't come across as new or relevant.

And untold legions of those galaxies must be hurtling in our direction. I am sure Krauss Scherrer would grant that likely possibility. The issue never comes up because it isn't relevant (unless they were traveling faster than their own light).

so if you think you are presenting an OBJECTION to them then you must not have understood.

We may need a thread about the cosmological event horizon[/color]

that is something you get after painstakingly fitting thousands of data points from supernovae, galaxy surveys, CMB, gammaray bursts etc etc., you fit the model to the data and you get a number of lightyears

the distance beyond which something can't get to you even if it travels at the speed of light

Those untold legions of galaxies hurtling towards us, if they are ever going to arrive and merge with us as you suggested, would have to be going so fast that they catch up with and pass by their own light which because it is only traveling at the speed of light will never reach us.

So I guess these legions of galaxies you say might be coming to merge with us must be made of TACHYONS or some exotic matter like MICKYONS. In any case I would not like to mess with them. They might not taste good.

My preference is for informed skeptics[/color] who understand what they are doubting and disagreeing with.

 

[ukmicky]

Those untold legions of galaxies hurtling towards us, if they are ever going to arrive and merge with us as you suggested, would have to be going so fast that they catch up with and pass by their own light which because it is only traveling at the speed of light will never reach us.

So I guess these legions of galaxies you say might be coming to merge with us must be made of TACHYONS or some exotic matter like MICKYONS. In any case I would not like to mess with them. They might not taste good.
.

Thankyou, I Understand what your saying, timescales ,speed of light understood .. my understanding will l get there one day,Its just a shame i can't hitch a ride on the tachyon express to speed things up.