The Search for Dark Matter and Dark Energy

mlimber writes "The New York Times Magazine has a lengthy article on dark matter and dark energy, discussing the past, present, and future. 'Astronomers now realize that dark matter probably involves matter that is nonbaryonic ["meaning that it doesn't consist of the protons and neutrons of 'normal' matter"]. And whatever it is that dark energy involves, we know it's not 'normal,' either. In that case, maybe this next round of evidence will have to be not only beyond anything we know but also beyond anything we know how to know.'"

Knowing Know

In that case, maybe this next round of evidence will have to be not only beyond anything we know but also beyond anything we know how to know.

I knew he was going to say that.

Re:Knowing Know

> > In that case, maybe this next round of evidence will have to be not only beyond anything we know but also beyond anything we know how to know.
>
> I knew he was going to say that.

As long as we're quoting Rumsfeld, "You do high-energy physics with the particle accelerators you have. It's not the particle accelerator you might want or wish to be able to build at a later time."

Can dark matter just be..

...big black holes that have already eaten everything around them? (i.e the "edges" of the universe)

..."in-transit" energy from 100,000,000,000 stars?

...large amounts of completely non-reflective dust and asteroids?

...a side effect of over-estimating the size of the universe? (i.e. stars like our 5 billions light years away don't exist anymore)

/real questions
//just curious..

Re:Can dark matter just be..

Large black holes are located at the center of galaxies, and their mass can be determined by examining rotation curves, etc. They are not dark matter candidates. Primordial black holes are not massive enough. There is some possibility that dark matter could be non-luminous dust, but there are some limits placed on observations of the comsic microwave background, which would have had to travel over 13 billion light years through such dust without being significantly attenuated.

The 'size' of the universe is an ill-defined question. We can only observe what's in our past light cone, and it is *that* universe which suffers from a budget shortfall of matter/energy.

Re:Can dark matter just be..

There is some possibility that dark matter could be non-luminous dust, but there are some limits placed on observations of the comsic microwave background, which would have had to travel over 13 billion light years through such dust without being significantly attenuated.

Galactic dark matter, which is required to explain the rotation curves of spiral galaxies, can be completely explained by baryonic dark matter, which would be at least partially dust.

Extra-galactic dark matter cannot be primarily baryonic. The baryon density of the universe is known from big bang nucleo-synthesis and the primordial H/He ratio, and is too small to account for extra-galactic dark matter. Therefore extra-galactic dark matter has no relation at all to galactic dark matter, as it cannot be made of the same stuff as galactic dark matter.

So there are at least two completely different, totally unrelated dark matter problems. One can and probably is solved by baryons. The other requires exotic particles or possibly exotic physics.

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So there are at least two completely different, totally unrelated dark matter problems.

You're right that the universal baryon density doesn't specifically constrain galactic dark matter. But Occam's Razor suggests there is only one dark matter problem. B

Re:Can dark matter just be..

It's true that there are multiple scales to the dark matter problem, and that our arguments for exotic dark matter apply on the extra-galactic scale. I don't think theorists seriously argue that baryons solve the galactic dark matter problem, however. The Bullet cluster result (Google for Sean Carroll's excellent piece on this) tells us that the dark matter in galaxy clusters can't be baryonic either (it interacts too weakly with ordinary matter). The numbers we have from various experiments add up best if even galaxies are dominated by dark matter halos.

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Hawking speculates that micro black holes are throughout the universe. That makes for a lot of places where dark matter could hide.

I can't find a paper in which he says this, so no citation.

Dark Matter Exists

Here [cosmicvariance.com] is an excellent article by Sean Carroll [preposterousuniverse.com] of the California institute of Technology that explains why all the suggestions of the parent post may not be correct.

Basically, what it says is that if two large clusters of galaxies went right through each other, and dark matter was really like the normal matter in the way the parent post suggests, we would get a different result from what would happen if dark matter was for real. Astronomers have discovered one such system and this provides conclusive evidence for the existence of dark matter.

How about ...

Very large bodies don't behave according to Newton. Very small bodies behave according to the rules of quantum physics, so it's clear that one law doesn't regulate every case. Dark matter/energy are just a fudge factor because we can't explain what happens without them, but that doesn't prove that they exist. All that is proven is that we don't understand what is happening.

Not really...

On the contrary, very large bodies are extremely well-approximated by Newton, as it is the slow-velocity, weak field limit of General Relativity. There is already good photographic evidence for dark matter in the form of colliding galaxies (do your Google work), and current observational evidence points pretty strongly towards dark energy in the form of a cosmological constant. While it's true we don't know what that means, it's not just a fudge factor.

Re:Not really...

You better tell John Moffat that very large bodies are extremely well-approximated by Newton so he can stop wasting his time on Tensor-Vector-Scalar [wikipedia.org] gravity.

Dark matter seems like far from settled science to me. But it always does amaze me how dark matter proponents tend to treat it's existence just like the followers of intelligent design treat God.

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I thought this too for a long time, but it seems that the only evidence for dark matter isn't just galactic rotation curves. I'm having trouble finding it through Google, but while I was studying astrophysics last year we were shown an image of a gravitati

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Very large bodies don't behave according to Newton. Very small bodies behave according to the rules of quantum physics, so it's clear that one law doesn't regulate every case.

Don't forget that this "law" is simply an equation based on observable evidence.

Nothing to see here...

Please move along.

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Clueless, humourless, moderator strikes again.

Dammit

And I just got my head around Quantum Physics... Now they are throwing this at me.

I think this might be one of those things I chose not to learn and just leave to someone else.

"Dark energy"

At this point, dark energy is really nothing more than a fudge factor. It's certainly nothing like the normal concept of energy. We don't even know if it's a cosmological constant or if it varies over time and space, let alone whether it's a property of sp

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There is NO question that that expansion of the universe is accelerating. According to General Relativity, the ONLY way this can be happening is if the universe is dominated by a species with a negative pressure. If you're not happy with the name dark en

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Sorry, but I think you completely ignored what the GP said and basically spouted a nonsensical stream of verbal diarrhoea which vaguely sounds like you know what the hell you are talking about when you actually don't. The GP was questioning the use of dark

GR does not work at Galactic Levels

GR does not work at Galactic Levels, so there is no question of it working at the Cosmic levels.

The real problem is MOND. If it did not exist then Dark Matter would be free to exist wherever it wanted. But with MOND the picture becomes more complex, now DM

Travel and other considerations?

Some questions that spring to mind:

If the grand majority of the 'stuff' in existence around the universe is matter that would be somewhat alien to our range of experiences, could this have an effect on inter-galactic travel? Would what we think it is so f

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Dark matter is not chemically interesting since, by definition, it doesn't interact with normal matter. Hence, it's unlikely to be 'useful' in any current fashion!

As far as whether it's dangerous -- if dark energy is a cosmological constant, it's a prop

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If it doesn't interact by the electromagnetic force, it cannot affect anything chemically. If it doesn't interact by the strong force, it cannot cause nuclear reactions. Even if it interacts by the weak force, the effect would be equivalent to the neutrino

Dazzling

That same year, Michael Turner, the prominent University of Chicago theorist, delivered a paper in which he called this antigravitational force "dark energy." ... "It really is very different from dark matter," Turner said. "It's more energylike."

That's an educated opinion, if I've ever heard one.

"Normal?"

If dark matter makes up most of the mass in the universe, wouldn't the kind of matter we're familiar with be the abnormal kind?

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If dark matter makes up most of the mass in the universe, wouldn't the kind of matter we're familiar with be the abnormal kind?

No, because we, as sentient beings on planet Earth define what "normal matter" is. Universe doesn't care at all.

My point being, d

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Wow. If I shall ever see something more politically correct than this statement, I can die happy.

I still want to know....

...what color dark matter is? God, I hope its not beige.

"beyond anything we know how to know"

Good to see that Rumsfeld has found a new job that lets him exercise his poetic skills.

Nibbler poop.

And whatever it is that dark energy involves, we know it's not 'normal,' either.

Nibbler knows what it is and from where it comes...

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I didn't know telecopes were that old. Is this a typo, and didn't they mean decades instead? If not, what did ancient telescopes do?

Hans Lipperhey [ezinearticles.com] invented the telescope in the late 1500s.

Re:Telescopes invented 400 years ago?

"Since the invention of the telescope four centuries ago"
I didn't know telecopes were that old. Is this a typo, and didn't they mean decades instead? If not, what did ancient telescopes do?

FooBarWidget, meet Galileo: Widely credited as the inventor of the modern telescope, in 1609.

Though, as with all major developments in human history, some accounts have him as merely improving on preexisting tech, whether copying the work of Lippershey from 40 years before, or even the possibly MUCH older designs of the ancient Persians.

So no, not a typo.

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Ancient telescopes were essentially spyglasses or binoculars, allowing one to see a great distance. And it's true that there is some evidence that arabs used such devices to study the stars even earlier. But it's generally regarded that the first telesco

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No, not a typo. You do remember a fellow named Galileo, right? Now, what was he famous for, I can't quite remember...

Why not look at the wiki page about the history of telescope? [wikipedia.org]

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No, it's not a typo. Although I would probably say even older. The old telescopes did much the same as new telescopes. They would allow a viewer to put his eye close to the ocular, and through it spot distant objects enlarged through the combination of len

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I believe the children are the future.
I bet this one could have a nice career in the ministry of truth.

Social politics

There's no money in globally regulating ESP. There's massive amount of taxpayer money to be siphoned off when funding and regulating particle accelerators, nuclear reactors, and telescopic arrays.

The primary driving motive behind 99% of everything which h

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Because they're not really wild hypotheses at all. You can OBSERVE the rotation curves of galaxies and see they don't match up with the estimates of the matter content. SOMETHING is there, so your only real quibble might be with the cryptic name 'dark ma

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The difference is that there are NOT many different studies confirming ESP happens. In fact, there are many studies arguing the contrary (particularly if you focus on studies from reputable sources). There are plenty of people who WANT ESP to be true, bu

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Which studies? How are they confirming "it happens"? What is the "it" they are confirming?

I'm sorry, but if you want to compare the top scientists of this world with a bunch of self-deceiving charlatans and quacks, and fail to find any difference, maybe

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Well, one (relatively paranoid) theory is that they don't want others getting the technology. "They" in this case is the powers-that-be. The US Government has been experimenting with Remote Viewing [wikipedia.org] for many years now. Some would say successfully so. But fr

Send them to the Randi foundation, then

Ok so i know this is off topic, but why are wild hypotheses like this taken so seriously when things like ESP/human mind altering random probability kind of things laughed at so widely when they actually have many different studies confirming it happens?

Heh. Well, then, just send them to the Randi foundation which still has a 1 million dollar prize for anyone who can prove anything like that. The requirements so far have been reasonable too, usually along the lines of having a scientific double-blind test. Nothing you wouldn't expect in normal science. Altering probabilities is even more straightforward, since then you just have to take a large enough sample and do some elementary statistics. So you'd think that if ESP or mind-over-matter or whatever floats your fantasy boat was that proven and working, someone would claim the prize already. But, nah, suspiciously so far what we've had were:

- bullshitters arguing about how unsound scientific testing is, and why they won't take part in it (sorry, if something is only perceived when the test subjects are told and persuaded what they should perceive, then it's probably just make-belief.)

- lame stage magician tricks

- various versions of some global conspiracy to suppress them (funny how noone suppressed them before, then. You'd think the conspiracy would then stop them from publishing books and making faked movies about it too, not just stop them from taking part in a controlled experiment.)

Etc.

Plus, Randi isn't the only one who came up empty so far. What fraudsters are quick to tell you, as if it were some proof of ESP existing, is that both the USA and the USSR were interested in it during the cold war. That much is true. Unsurprisingly, since for example transmitting a message to a submarine by a mean that's (A) not blocked by water or rock, hence receivable from any depth or hole, and (B) impossible to intercept, is any army's or navy's wet dream. What they conveniently ommit there is that both the USA and the USSR, and a few others for that matter, failed to get any results with it.

By contrast, the people with these physics hypotheses tend to actually have some verifiable/falsifiable data, and they give it to you up front. If they did just bullshitting and handwaving like the ESP gang, we wouldn't take them seriously either.

Re:Questions from a B- physics student

The best way to determine the matter content of the universe is through observations of the cosmic microwave background (CMB). The properties of the plasma that emitted the CMB are well known and be used to predict temperature anisotropies (variations) in it. These show up as peaks and troughs at different angular scales. We know approximately how far away the CMB is in terms of redshift (z ~ 1100... really far!), so these angular measurements give us a distance scale. In a curved universe, the peaks and troughs appear at different angles, whereas those observed are consistent with a flat universe. A flat universe MUST have a certain energy density, but the observed baryon density only accounts for about 4% of that.

This could all be accounted for by dark matter save for the observations of Type 1A supernovae which indicate accelerating expansion, and this requires domination by a state of matter with negative pressure, and this is what's been coined dark energy.

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you realize the stars in the galaxy only account for less than 1% of the mass required to hold the galaxy together

You forgot Step 7.5: "Assume that we know everything about all of the forces which hold galaxies together on the astronomical scale."

Same point

But he was using it in the opposite direction. The parent asked,"How do we know that dark matter isn't just blah blah blah", and the AC replied,"Because we can calculate blah blah blah", and I pointed out that perhaps the calculations were wrong.

I'm not s

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To answer your first question "How do they know that the matter is not accounted for?", here's how we do it:

Step 1. Pick a galaxy
Step 2. Determine its distance using variable stars (stars that change in brightness in well-known methods).
Step 3. Determine the absolute magnitude of the galaxy (how bright it would appear if it was a fixed distance away).
Step 4. Determine how much total mass all the stars in that galaxy have in order to provide that brightness.
Step 5. Observe the doppler shift in the light from the edges of the galaxy (the side rotating toward you will appear bluer than normal, while the side rotating away from you will appear redder that normal) to determine the rotational speed of the galaxy.
Step 6. Determine how much mass must be in the galaxy in order to provide the necessary centripetal acceleration to create the observed rotational speed.
Step 7. Compare answers from Step 4 and Step 6.
Step 8. Smack yourself in the head when you realize the stars in the galaxy only account for less than 1% of the mass required to hold the galaxy together.

That is all well and good but it may be that the need for "dark energy" and "dark matter" may be the result of sloppy science. If scientists cannot tell the difference between a distant giant galaxy and a nearby dwarf galaxy, how can you believe a word th

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That is all well and good but it may be that the need for "dark energy" and "dark matter" may be the result of sloppy science. If scientists cannot tell the difference between a distant giant galaxy and a nearby dwarf galaxy, how can you believe a word they say about missing mass?

Doesn't this article quite clearly show that people can tell the difference, however for this particular galaxy (presumably unremarkable and not very well observed, given that apparently nobody has taken a measurement of the red-shift for the past two dec

God is not...

God...

* is not "a him".
* is not even "an it".
* doesn't even have a definition that makes sense.

So...

* how can you "believe" in something you cannot accurately describe?
* how can you "believe" in an idea that doesn't make sense?
* Does believing that blue is

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So long as they don't create a black hole somewhere. :)

It wouldn't matter, tiny black holes go away on their own. The derivation is not listed here (it's just stated) but I can assure you that it's loads of fun to calculate.

http://en.wikipedia.org/wiki/Black_hole#Black_hol e s_and_Earth [wikipedia.org]