Could Dark Energy be Dark Matter cooling?

[Reverend2010]

Hi Clever People,

I've been watching an amazing program about dark matter, dark energy and dark flow. I thought there were questions that should have been asked, but then I'm a website designer - what do I know!

So... as an interested non-physicist, my apologies if this question is ridiculous...

If Dark Energy is what is causing the 'empty' regions of deep space to expand / accelerate, could it be due to Dark Matter cooling down? Or heating up? Or perhaps even multiplying - like cell-division?

I mean, if we're not quite sure what Dark Matter is except that it behaves totally differently from anything else, could it behave as above?

 

[mathman]

Your idea is completely far out. Dark matter is stuff and it is manifest by gravitational effect only. Dark energy is a complete mystery - accelerating the expansion of the universe.

 

[Naty1]

Dark energy is also called the cosmological CONSTANT...it is a constant is all frames of reference and so cannot be heat energy...you might gain an understanding of this by reading about the Unruh effect...
how Einstein figured out it is a constant I don't know, but there seems to be general agreement it IS a constant...

You might find some interesting reading here.

http://en.wikipedia.org/wiki/Dark_energy

Of course, most of the time most physicsts have over history been proven WRONG...as new theories are developed and tested...so who knows...

 

[typical guy]

Dark energy is also called the cosmological CONSTANT...it is a constant is all frames of reference and so cannot be heat energy...you might gain an understanding of this by reading about the Unruh effect...
how Einstein figured out it is a constant I don't know, but there seems to be general agreement it IS a constant...

You might find some interesting reading here.

http://en.wikipedia.org/wiki/Dark_energy

Of course, most of the time most physicsts have over history been proven WRONG...as new theories are developed and tested...so who knows...

How certain are we that the cosmological constant is the same in all regions of space? In other words, is it possible that this "dark flow" that's recently been in the news is related to part of the universe expanding faster than neighboring regions? It seems logical to me that if there was inflation in the early universe then there is the possibility that regions of space are still undergoing inflation although these regions may be beyond our observable universe.

 

[PhanthomJay]

Now I was led to believe, perhaps mistakenly, about 10 years ago, as proposed, I think, by the M-Brane theorists, that Dark Matter (or Dark Energy, which is just a different manifestation of dark matter, per E=mc^2) was a result of ultra high frequency short wave gravity waves escaping, thru the miniscule (in the order of the Planck length) higher order spatial dimensions of spacetime, from matter of another parallel universe. Is Brane Theory (M-Theory) dead??

 

[mathman]

Dark energy is not a manifestation of dark matter. Even calling it energy is misleading. It seems to be related to the cosmological constant. Einstein made it up as a fudge factor to keep the universe static - which people believed at the time. He later called it his biggest blunder after the expansion of the universe was discovered.

 

[PhanthomJay]

So what do you call the energy equivalence of dark matter, per E=mc^2??

 

[A Janitor]

So what do you call the energy equivalence of dark matter, per E=mc^2??

Calling it Dark Matter is misleading, it would be better named 'missing matter'. Our current theories of gravity do not hold up with the current mass in the universe. So either our theories of gravity are incorrect, or there is 10 times the amount of matter in the universe then there appears to be. Obviously we aren't going to go back the the drawing board with gravitational theories, so abracadabra, dark matter.

In other words there is no 'dark matter' we don't know what we're missing and we just call it dark matter.

So you can't actually have statistics or theories for it, since we haven't discovered it.

 

[PhanthomJay]

I don't care if you call it dark matter or missing matter, I'm just wondering if some clever physicist has coined the term for the energy equivalence of the 'missing' matter , per E=mc^2, if in fact, the dark matter has mass (does it?) and obeys the laws of Physics (does it?).

 

[mathman]

Dark matter has mass - that is about all we know. It is detectable by its effect on galaxies and galaxy clusters.

 

[PhanthomJay]

So if it has mass, it has energy, assuming it obeys the Universal Law of Physics, as we know it. Maybe we don't know if it obeys these laws? If it does, maybe i'll coin a word for it and become famous. Incidentally, is dark energy the latest catchword foo what used to be called 'vacuum' energy?

 

[DarkStar7]

Dark energy looks suspiciously like inflation generated by all those mysterious particles spotaneously popping out of quantum foam uncertainties.

 

[A Janitor]

Dark matter has mass - that is about all we know. It is detectable by its effect on galaxies and galaxy clusters.

We do not 'know' dark matter has mass. We 'know' that there is a unexplainable force causing more gravity then the known mass acounts for. Really it is much more likely that our theories on gravity are wrong then an invisible all powerful 'mass'-ive force holding the universe together.

That sounds more religious then science, but the human race has a tendency to make up stuff when they can't explain what they see. That is where dark matter comes from, the human imagination. So asking if the imaginary mass has energy has a very simple answer: It has imaginary energy!

All this energy comes from unicorns that live in an invisible planet made of dark matter.

Ain't science great!?

 

[mathman]

Really it is much more likely that our theories on gravity are wrong then an invisible all powerful 'mass'-ive force holding the universe together.

Since there is no evidence (except what may be happening inside black holes) that general relativity is wrong, your assertion seems less likely than the existence of dark matter, particularly since astronomers seem to be able to get a picture of its distribution.

 

[A Janitor]

Since there is no evidence (except what may be happening inside black holes) that general relativity is wrong, your assertion seems less likely than the existence of dark matter, particularly since astronomers seem to be able to get a picture of its distribution.

Umm... What is your evidence that my invisible unicorn theory is wrong?

 

[haael]

We do not 'know' dark matter has mass. We 'know' that there is a unexplainable force causing more gravity then the known mass acounts for. Really it is much more likely that our theories on gravity are wrong then an invisible all powerful 'mass'-ive force holding the universe together.

Actually, we know a bit more about dark matter. It doesn't have pressure (i.e. it doesn't interact electromagnetically), and it concentrates around galaxies. Different gallaxies have different percentage of dark matter content, so there definitely is something around we don't see - not just our theories are wrong.
So, when we already have started to measure dark matter density, it is not very wise to claim it was created by imaginary unicorns.

About dark energy - we don't even know whether the universe is accelerating or slowing down! Our guessing is based on supernova theories (we don't know if they are correct) and actual experiments are so few, that saying anything about the Universe shape is way too soon.
The only thing we know - galaxies are red-shifted. Anything else is just imaginary unicorns. We don't know the relation between distance and red shift, because we can't determine neither.

So, don't get depressed. We can bake almost any theory about Universe geometry and no one can proove us wrong yet :).

 

[Ich]

The only thing we know - galaxies are red-shifted. Anything else is just imaginary unicorns.

Come on. Somehow you missed the last 80 years?
There's SN1a, WMAP, BAO, nucleogenesis... all of which give independent evidence for a coherent picture. It's not like LCDM depends on a single, poorly understood observation.

 

[Janus]

We do not 'know' dark matter has mass. We 'know' that there is a unexplainable force causing more gravity then the known mass acounts for. Really it is much more likely that our theories on gravity are wrong then an invisible all powerful 'mass'-ive force holding the universe together.

You are vastly understating the body of evidence that exists in support of the dark matter model.

It's not as if nobody has tried to come up with alternative theory of gravity to explain it, it just that every promising one has been shot down by new observation that can't be made to fit. The DM model has not been shot down by these new observations which is why it still survives.(in fact, each new observation seems to increase the likely-hood of its being correct)

 

[JcX]

The idea of dark matter comes from mathematical model of gravitational force.
Scientist suggests that there must be matter that we don't see in our universe, maybe between galaxies that exert the gravitational effects that we feel.
The content/percentage of dark matter is chosen to fit and balance all the gravitational effects that has been observed in the universe (expansion of universe, perturbation of orbits, etc).
We predicted the existence of them, but we can't be really sure of what dark matter is.

Some theories such one those who suggest multi-dimensional universe says that dark matter is actually some matters in other universe. String Theory says that only gravitons are able to escape from the membrane of the dimension, hence only gravitational forces are able to penetrate between dimensions. Living in this dimension, we can feel the gravitational effects from other dimension, but we don't see them, that's why we called them Dark Matter.
That's one of the explanation that I liked a lot... although... it's hard to imagine.

 

[haael]

Come on. Somehow you missed the last 80 years?
There's SN1a, WMAP, BAO, nucleogenesis... all of which give independent evidence for a coherent picture. It's not like LCDM depends on a single, poorly understood observation.

All our calculations of distance are based on supernova theories. All this "acceleration" may as well be an error in our understanding of star dynamics.

 

[DarkStar7]

The basic ideas of the standard model are well established. However what is the most ridiculous part of this standard mode? It is inflation! The fundermental properties of the initial start is very well understood, then we have our universe. It seemed so natural to think of a process that gets from an
infinitely small high energy point towards the universe we see today.Hey presto we have inflation. How can any scientists be taken seriously when they come up with such a crazy conclusion. They'll have us believe that spacetime expanded faster than the speed of light to get from the big bang to where we are today with a total disregard of Einsteins ideas.
Then we have Quantum theory that says that spacetime is a neutral summation of positive and negative forces. The uncertainty principle allows these forces at extremely small scales to momentarily obtain large energies. There are claims that particles within spacetime can briefly pop out of absolutely nothing.
Now we have dark matter and dark energy that fills all space which no one understands. The phenomena affects the speed of galaxies and seems to have a repulsive force.

Surely it's about time that the ideas of Sir Fred Hoyle. Who I consider should be recognised as the greatest scientist since Albert Einstein should have his last theory of Quasi Steady State theory re-examined seriously. It brings together the great ideas of the Standard Theory and the best of the Steady State Theory together.

To me it is obvious... the dark energy is inflation the byproduct of universe full of MINI BANGS. Constant creation occurring within the fabric of space, existing within a dynamic Universe with no beginning or end. A universe where matter and spacetime are curved back from the largest scales.. to feul the energies within the quantum world at the smallest scales.

Dark Energy IS inflation. It has to be.

 

[A Janitor]

You are vastly understating the body of evidence that exists in support of the dark matter model.

It's not as if nobody has tried to come up with alternative theory of gravity to explain it, it just that every promising one has been shot down by new observation that can't be made to fit. The DM model has not been shot down by these new observations which is why it still survives.(in fact, each new observation seems to increase the likely-hood of its being correct)

dark matter persists because it is not a theory. It has no actual attributes other then a mathmatical model. What is it? By definition it is 'whatever is causing the unknown gravity'. I do not think that a 'whatever' should be considered a viable scientific theory. Again how can you prove my invisible unicorns wrong. Just because there is evidence that other theories are wrong does not mean my unicorns do not exist. In fact since other theories are wrong my unprovable unicorns are more likely to be right.

Come on. Somehow you missed the last 80 years?
There's SN1a, WMAP, BAO, nucleogenesis... all of which give independent evidence for a coherent picture. It's not like LCDM depends on a single, poorly understood observation.

You have mentioned other observations that point to the fact that there is something we do not understand. I am not saying that 'something' isn't causing what we see, I am saying it is not 'dark matter'. Dark matter is an undefined identity, it just sounds scientific but has as much base as my invisible unicorn theory. Why couldn't invisible unicorns be causing these effects? Nothing in science actually disallows the existence of invisible unicorns, what makes invisible unicorns less viable then dark matter.


BTW I am not a multiverse theory fan either, nor a string theory fan, for the same reasons. Mathimatical models should explain observations not create them.

 

[bapowell]

Really it is much more likely that our theories on gravity are wrong then an invisible all powerful 'mass'-ive force holding the universe together.

Really? Why do you say this? Most modifications to gravity theory don't offer plausible alternatives to dark matter. The bullet cluster http://en.wikipedia.org/wiki/Bullet_Cluster practically closed the book on alternative theories of gravity in favor of particulate dark matter. [/QUOTE]

That sounds more religious then science, but the human race has a tendency to make up stuff when they can't explain what they see. That is where dark matter comes from, the human imagination. So asking if the imaginary mass has energy has a very simple answer: It has imaginary energy!

All this energy comes from unicorns that live in an invisible planet made of dark matter.

Ain't science great!?

The only thing stupid about dark energy is its name. We know that vacuum energy can lead to accelerated expansion of spacetime. This is what happens during inflation. What's happening now is fundamentally no different. You should learn a bit more about the science before you make such heavy-handed opinions about things.

 

[bapowell]

All our calculations of distance are based on supernova theories. All this "acceleration" may as well be an error in our understanding of star dynamics.

This isn't correct. There is evidence from the cosmic microwave background that the present day universe is undergoing accelerated expansion.

 

[bapowell]

Hey presto we have inflation. How can any scientists be taken seriously when they come up with such a crazy conclusion. They'll have us believe that spacetime expanded faster than the speed of light to get from the big bang to where we are today with a total disregard of Einsteins ideas.

You clearly don't have a good grasp on cosmology, let alone inflation. I don't quite understand where your hubris comes from, but I'll try to explain things. The first lesson is that Einstein's special relativity applies locally to inertial observers. It does not apply to cosmology. For this we need general relativity. Here, it's perfectly OK for space to expand at superluminal rates. The stuff in the universe must of course obey v < c locally, and all of it does -- even during inflation.[/QUOTE]

Then we have Quantum theory that says that spacetime is a neutral summation of positive and negative forces.

Something doesn't seem right with this. Could you elaborate?

The uncertainty principle allows these forces at extremely small scales to momentarily obtain large energies. There are claims that particles within spacetime can briefly pop out of absolutely nothing.
Now we have dark matter and dark energy that fills all space which no one understands. The phenomena affects the speed of galaxies and seems to have a repulsive force.

Right. Might the dark energy that nobody understands be the large energies associated with the quantum vacuum??

Dark Energy IS inflation. It has to be.

It could be!

 

[bapowell]

You have mentioned other observations that point to the fact that there is something we do not understand. I am not saying that 'something' isn't causing what we see, I am saying it is not 'dark matter'. Dark matter is an undefined identity, it just sounds scientific but has as much base as my invisible unicorn theory. Why couldn't invisible unicorns be causing these effects? Nothing in science actually disallows the existence of invisible unicorns, what makes invisible unicorns less viable then dark matter.

Wow. I am not saying this to be rude, but you really don't know what you are talking about. You really do have as much of an understanding of dark matter as you do about invisible unicorns. But that doesn't mean the rest of us do. Don't conflate your misunderstanding of the concepts with a problem of the science.

 

[graybass]

I too am a layman, but doesn't gravitational lensing of deep space objects greatly point in the direction of dark matter? Isn't the amount of lensing observed absolutely say there is missing matter between here and threr?
JMHO

 

[ManyNames]

Hi Clever People,

I've been watching an amazing program about dark matter, dark energy and dark flow. I thought there were questions that should have been asked, but then I'm a website designer - what do I know!

So... as an interested non-physicist, my apologies if this question is ridiculous...

If Dark Energy is what is causing the 'empty' regions of deep space to expand / accelerate, could it be due to Dark Matter cooling down? Or heating up? Or perhaps even multiplying - like cell-division?

I mean, if we're not quite sure what Dark Matter is except that it behaves totally differently from anything else, could it behave as above?

God only knows, i'd have to write up the equations if there are many asking for it :)

The cooling of particles must result in a heavy state of particle with inertial mas, however, it's cooling would be quicker due to gravitational radiation and its relation to the kinetic energy of the system.

Also, dark energy does not make space expand. By logic, dark energy was created alongside the rest of the Sta. Model - during big bang. That is of course it was something which became an emergent energy source by reasons which our mathematical constructiions could even fathom...?

Who known... But i don't know this convo will get anywhere else.

 

[bapowell]

Also, dark energy does not make space expand.

It does indeed. Dark energy with an equation of state w \leq -1/3 will cause accelerated expansion of space.

 

[muppet]

Cosmology's not my thing, but insofar as I claim to know anything about it, I'd just like to back up bapowell here.
To elaborate on the above post, you can reconcile GR with the observations if you assume the existence of a substance that exerts a negative pressure (assuming that densities are positive!); it then plays the role of a "cosomological constant" in the Einstein equations.

The question then arises: what could behave in such a pathologically strange way? Regarding the connection with the vacuum energy of quantum field theory; as I understand it back-of the envelope calculations yield an estimate that is ~120 orders of magnitude too large to be commensurate with the observations; I've heard it been described as "the worst theoretical prediction in the history of science".

 

[A Janitor]

Wow. I am not saying this to be rude, but you really don't know what you are talking about. You really do have as much of an understanding of dark matter as you do about invisible unicorns. But that doesn't mean the rest of us do. Don't conflate your misunderstanding of the concepts with a problem of the science.

I think you are misunderstanding what I am saying. The theory of Dark Matter does not define what dark matter is, other then a gravitational phenomena. If I were to ask 'What is dark matter' or ask anything along the lines of 'can you touch it, see it, smell it, hear it' any answers would be personal speculation, and not based on observation. Calling it dark matter doesn't give it properties. In other words I could just as easily say God is creating the extra gravity (which was why I referenced it as religious in nature).

If you have a undefined object causing effects you can redefine it with every new observation. I would not call this a scientific theory. I would call it a mathematical model, or I would call it a 'placeholder' theory.

Assuming that I know nothing on this topic, what about the rising number of cosmologist that agree with my statements. Discover Magazine recently published an article that agreed with these views.

Really?
The only thing stupid about dark energy is its name. We know that vacuum energy can lead to accelerated expansion of spacetime. This is what happens during inflation. What's happening now is fundamentally no different. You should learn a bit more about the science before you make such heavy-handed opinions about things.

I was not referring to the dark energy theory, I was referring to the original question that is the title of this thread. I may be wrong (and please cite a non-wikipedia article if I am) but I do not think that dark matter theory draws any sort of direct correlation to dark energy.

Also please stop using Ad Hominem attacks against me, you assume I know nothing because I am saying something against the standard view. Even if I am wrong it is the people who question theories that progress science.

I am also very open to listening to reasoning, but you have not yet provided any argument other the Ad Hominem attacks.

My theory is that you are (at most) a college undergrad, your cockiness and citations to wikipedia give away your lack of professionalism.

"College isn't the place to go for ideas."
-Hellen Keller

 

[bapowell]

Assuming that I know nothing on this topic, what about the rising number of cosmologist that agree with my statements. Discover Magazine recently published an article that agreed with these views.

I agree that when dark matter was first hypothesized, nothing was understood about it. You're right in saying that it may well have been God messing with gravity. However, over the past 2 decades, there has been much learned about dark matter. Dark matter is well motivated as a thermal relic in most generic models of the early universe, of the correct mass and interaction strength to account for today's observations (galaxy rotation curves, cosmic microwave background acoustic oscillations, things like the Bullet Cluster). BTW, I cited wikipedia because it's a simple account of what the Bullet Cluster is. Nothing more. I felt that your comments were not only counter to the standard view held by cosmologist, but simply out of date. Much is understood about dark matter, and there are consensus views that are not debated within the community. Also, I'd be interested in the Discovery reference, if you have it.

I was not referring to the dark energy theory, I was referring to the original question that is the title of this thread. I may be wrong (and please cite a non-wikipedia article if I am) but I do not think that dark matter theory draws any sort of direct correlation to dark energy.

That's right, there's no necessary connection.

Also please stop using Ad Hominem attacks against me, you assume I know nothing because I am saying something against the standard view. Even if I am wrong it is the people who question theories that progress science.

I assumed you were either misinformed or negligent with regards to your research, not because you advocated an alternative view, but because you were claiming that scientists know nothing about dark matter. That is absolutely untrue. I'm not claiming that you know nothing, but your statements suggest this.

My theory is that you are (at most) a college undergrad, your cockiness and citations to wikipedia give away your lack of professionalism.

I'm a PhD cosmologist who studies inflation. I'd be happy to discuss specifics of the dark matter debate, but I don't respond well to sweeping generalizations made by people who don't appear to know the facts. Again, I'm not saying you don't know the facts, but your comments so far suggest otherwise.

 

[twofish-quant]

I think you are misunderstanding what I am saying. The theory of Dark Matter does not define what dark matter is, other then a gravitational phenomena.

Yes it does.

1) it's cold
2) it doesn't interact with the strong nuclear force or electromagnetism
3) it doesn't strongly interact with ordinary matter
4) It doesn't seem to exactly match the distribution of non-dark matter

We've made a lot of progress understanding what it is by ruling out what it isn't.

If I were to ask 'What is dark matter' or ask anything along the lines of 'can you touch it, see it, smell it, hear it' any answers would be personal speculation, and not based on observation.

No they wouldn't. There are dark matter detection experiments going on right now. If you don't see it in a certain way, you understand what you are dealing with. Again, you understand what something is by understanding what it isn't.

If you have a undefined object causing effects you can redefine it with every new observation. I would not call this a scientific theory. I would call it a mathematical model, or I would call it a 'placeholder' theory.

Yes it is. Theories are meant to be disproved. If dark matter is X, they we should see Y. We do not see Y, therefore it can't be X.

As far as professional opinion goes, dark matter is winning the horse race right now against modified gravity theories. Modified gravity hasn't been knocked out yet, but it is staggering a bit.

My theory is that you are (at most) a college undergrad, your cockiness and citations to wikipedia give away your lack of professionalism.

I have a Ph.D. in astrophysics, and I think that wikipedia is great. Wikipedia works more or less the way that science works. Yes, you shouldn't believe everything you read in wikipedia, but then again you shouldn't believe everything you read anywhere.

 

[ViewsofMars]

I want to chime in on the issue about Dark Energy. I read an article dated March 2, 2010, "UA Receives Contracts Worth $6M to Support Quest for Dark Energy" by Daniel Stolte.

Two University of Arizona research and development groups were selected to develop and manufacture key technology for the first major undertaking to investigate the mystery of dark energy in the universe.

UA's Imaging Technology Laboratory, a research group within Steward Observatory, and the Optical Fabrication and Engineering Facility at the College of Optical Sciences will provide image recording devices and the heart of the optical system used for imaging, respectively, for the Hobby-Eberly Telescope at the McDonald Observatory, which is operated by the University of Texas, Austin.

The additions are part of outfitting the world's fourth largest optical telescope with an array of new instruments to analyze the light from distant galaxies in an effort to understand the nature of dark energy. Scientists have known for a while that dark energy exists, but so far, nobody has been able to come up with an explanation of what it is.

During the Hobby-Eberly Telescope Dark Energy Experiment, or HETDEX, the telescope will search a large area of the sky that encompasses most of the Big Dipper constellation. This region is far above the plane of the Milky Way galaxy, which is filled with clouds of gas and dust that block the view of distant galaxies. During three years of observations, UA's imaging sensors will collect the faint light from at least one million galaxies that are between nine billion and eleven billion light-years away. The data about their movement, size and precise locations with respect to one another will then be computed into the largest three-dimensional map of the universe ever produced.

The Optical Fabrication and Engineering Facility at the College of Optical Sciences was awarded a $4 million contract to devise and build an extremely complex optical device known as a wide field corrector.

"The wide field corrector broadens the telescope's view into space," explained Martin Valente, who directs the facility, "enabling it to survey vast swaths of the sky in a relatively short amount of time."

Ever since the Big Bang, the universe has been expanding and galaxies are moving away from each other. Galaxies attract each other through their immense gravitational forces, which should slow down the expansion of the universe. Instead, the expansion of the universe has sped up over time. This acceleration is attributed to some unknown force – dark energy – counteracting the galaxies' gravitational pull, causing them to spread out deeper into space.

The map of galaxies produced by the HETDEX survey will allow astronomers to measure how fast the universe was expanding at different times in its history. Changes in the expansion rate will reveal the role of dark energy at different epochs. Various explanations for dark energy predict different changes in the expansion rate, so by providing exact measurements of the expansion, the HETDEX map will eliminate some of the competing ideas.
Read more:
http://uanews.org/node/30362

And here is a snippet from HETDEX website though I encourage interested parties to explore this incrediable website:

The Solution
The first step is to measure the effect of dark energy on the Universe with very high precision--specifically, to measure exactly how the Universe has expanded over time. When we look at distant objects, due to the finite speed of light, we are able to see back in time. We call this look-back time. Hence, we can measure the properties of the Universe back in time, by observing more and more distant galaxies and supernovae. Using the supernovae that first demonstrated the existence of dark energy, we can probe the size of the Universe to 9 billion years. At greater look-back times, detection becomes too difficult.

The ideal tracer of the Universe's expansion history, all the way back to 12 billion years (nearly 90% of the age of the Universe), is the large-scale distribution of galaxies. As the Universe expands, the distance between galaxies increases. There are characteristic patterns in the distribution of galaxies which can be measured. These patterns increase in scale as the Universe expands. Therefore, by comparing the size of the patterns in the distribution of galaxies at different look-back times, one can measure the expansion of the Universe over cosmic time.

HETDEX

Uncovering the patterns in the distribution of galaxies requires a survey to map out the positions of a million galaxies in a volume ten times larger than any survey to date. This cannot be done with any existing instrument on any large telescope, but it can be achieved on the Hobby-Eberly Telescope with the new VIRUS instrument (HET).

The HET is a premier telescope for surveys, and it is an ideal telescope to solve the problem of dark energy. Run by a small consortium, it ranks among the world's largest telescopes, with an effective aperture of 9.2 meters (360 inches). Due to its revolutionary design, it was constructed at just 20% of the cost of comparable telescopes.

HETDEX promises the largest ever galaxy survey. Among several studies now planned targeting dark energy, none will obtain the early times HETDEX is designed to probe. Additionally, with funding, HETDEX can be complete within 8 years, sooner than other surveys. These factors promise to make HET a dominant player in the endeavor to understand dark energy.
http://www.as.utexas.edu/hetdex/

What exciting times we live in! I love telescopes.

 

[Reverend2010]

This is amazing - I posted this question 2 years ago and then forgot about it - and just found it in my bookmarks... I love that a simple question has generated 3 pages of good debate from clever people... Job done!

Thanks all for the replies.

 

[twofish-quant]

In the career's thread I always mention that I've been fascinated with the politics and economics of astrophysics, and the HET is one of the things that got me interested in that.

Let's go back to the 1980's. Oil was super expensive, Texas was getting flooded with money, and people at the universities there were talking about using oil money to make some big enormous telescopes. Then the boom collapsed, and people looked at the money available and there wasn't enough to build the perfect telescope.

So then what people did was to figure out how to maximize science and minimize cost. Most of the cost of a telescope turns out to be in the mount. You have a multi-ton piece of metal and then you have this big machine to point the telescope at what you want to point it at. Another big cost is the internal mirrors. In order to get an image you need to mount a giant mirror inside that that makes thing expensive.

So people figure out that instead of making a big expensive general telescope, you could with very little cost make a telescope that's really cheap but it good at one thing, which is doing spectroscopic surveys. Now if you are doing galaxy surveys, the cool thing is that you don't care where you point your telescope at. You point your telescope at some random part of the sky and if it's not getting blocked by the Milky Way, then any random part of the sky is as good as any other random part.

So you remove the ability to point the telescope everywhere and that saves $$$$$ since you don't have huge motors.

Now you are doing spectroscopy. That means no images. Without images you can then use fiber optics to move light from the top of the telescope down to the instruments Again lots of $$$$ savings.

Since you don't have to spend money pointing the telescope or dealing with internal mirrors you can use some of that to make the big light bucket at the bottom bigger.

What's really cool is that all of these decisions were being made in the late-1980's and early-1990's before people had even dreamed of dark energy. It turns out that HET is the perfect instrument for studying dark energy, but none of the designers realized that.

 

[nikkkom]

Gosh. It's really not *that* complicated.

We already know one "dark matter" particle: it's neutrino. It has mass, but does not interact electromagnetically. If a narrow beam of one solar mass of neutrinos would fly through Solar System, we wouldn't see it, but sure as hell we will feel its attraction. (Thankfully, neutrinos never travel in such humongous massive tight beams)

So dark matter not merely CAN exist, we know it DOES EXIST, at least a part of it, and we are pretty confident what it is. It is not mysterious.

But since we also are pretty sure that (known) neutrinos alone can't explain what we see, it's not too difficult to postulate that there exist other similar particles: ones which have mass, but do not interact electromagnetically. To match observations, we postulate that these particles are more massive than (known) neutrinos.

So why are you guys so freaked out by "mysterious" dark matter? Are you feeling the same about neutrinos? I think not...

 

[twofish-quant]

But since we also are pretty sure that (known) neutrinos alone can't explain what we see, it's not too difficult to postulate that there exist other similar particles: ones which have mass, but do not interact electromagnetically. To match observations, we postulate that these particles are more massive than (known) neutrinos.

It is in fact *very* difficult to postulate that there are other similar particles.

Basically you start off with one equation that describes how particles behave. It's very, very difficult to add a new particle without causing the equation to either predict things that we don't see or be inconsistent theoretically.

One particular problem with massive particles is that massive particles will decay into less massive particles unless there is something that prevents that from happening. For example, there is something called baryon number. Because the proton is the lightest particle with a non-zero baryon number, it can't decay to something lighter. However heavier particles can and do decay into the proton.

So if you invent a "heavy particle" you are going to have to mathematically describe how that particle interacts with other particles, and this is rather difficult to do without tripping of something that we already know

So why are you guys so freaked out by "mysterious" dark matter? Are you feeling the same about neutrinos? I think not...

Everything is easy until you know why it's hard. The bottom line is that you just can't randomly add a particle. Adding a particle is like adding an element in the periodic table. If you want to add something at the end, no problem. If you want to add something between carbon and nitrogen, big problem.

You can graph the known particles and the form a nice chart. There is no obvious place to put another particle. You can assume that there is a heavier neutrino, but that means you need a heavy quark and all of that violates experiments that say that you have only three families of particles.

 

[nikkkom]

It is in fact *very* difficult to postulate that there are other similar particles.

Basically you start off with one equation that describes how particles behave. It's very, very difficult to add a new particle without causing the equation to either predict things that we don't see or be inconsistent theoretically.

You did not understand me.

I am not saying that to add a particle to the Standard Model is very easy. I am somewhat familiar with the math involved, I know that it's not trivial.

I am saying that some people seem to think that postulated dark matter is a very unusual kind of matter we never saw before, and thus they find it hard to believe it may be a viable theory. But dark matter is not something unlike we ever saw before - neutrinos are similar to it, and we know about neutrinos for what, 80 years already.

 

[nikkkom]

You can graph the known particles and the form a nice chart. There is no obvious place to put another particle. You can assume that there is a heavier neutrino, but that means you need a heavy quark and all of that violates experiments that say that you have only three families of particles.

Well, how about right-handed, so-called "sterile neutrinos"? Seesaw mechanism which gives them large mass? People are working on such models right now...

 

[twofish-quant]

I am saying that some people seem to think that postulated dark matter is a very unusual kind of matter we never saw before, and thus they find it hard to believe it may be a viable theory.

Dark matter passes the principle of "least weirdness". It's weird but everything else is even weirder.

But dark matter is not something unlike we ever saw before - neutrinos are similar to it, and we know about neutrinos for what, 80 years already.

We know that ordinary neutrinos are *not* similar to dark matter. You can invent something about weird neutrinos. One other thing is that even if you restrict yourself to baryonic matter, most of that material is dark.

Well, how about right-handed, so-called "sterile neutrinos"? Seesaw mechanism which gives them large mass? People are working on such models right now...

Yes, exactly...

http://arxiv.org/pdf/1102.4774.pdf

http://arxiv.org/abs/1204.3902

But the point here is that you just can't invoke a new particle. Every time you invoke a new particle you have to do a ton of work to justify that new particle.

 

[nikkkom]

We know that ordinary neutrinos are *not* similar to dark matter.

Are you argumentative a-hole or something?

I am not going to argue what level of similarity is required to qualify for word "similar".

If you think neutrinos are sufficiently different from hypothetical dark matter particles (they have different mass! wooo hooo) so that word "similar" can't be applied, feel free to think that way. I don't care.

One other thing is that even if you restrict yourself to baryonic matter, most of that material is dark.

"Dark matter" is a misnomer. It is not in fact dark, it seems to be transparent. Baryonic matter is not. Even as a dilute gas, it is detectable by observations in EM.

 

[Chronos]

Neutrinos are very 'hot' compared to dark matter. The only DM models that appear to work require non-relativistic velocities.

 

[twofish-quant]

Calm down.

If you think neutrinos are sufficiently different from hypothetical dark matter particles (they have different mass! wooo hooo) so that word "similar" can't be applied, feel free to think that way. I don't care.

This is the problem with these sorts of arguments. You see two things and they are "similar". I see the same two things and they aren't.

Now there are reasons why neutrinos don't look the same as other particles to me is that I did a lot of research on neutrino radiation hydrodynamics. To me saying that the dark matter particle and neutrinos are similar because they both interact with the weak force only is like saying that a bowling ball and an orange are similar because they are both round. This doesn't make much sense to a professional bowler or an orange grower.

Now I'm not going to get annoyed if someone says that things look similar. Just don't get too annoyed at me if I tell you that they don't look similar to me, because they don't.

Remember ***you*** are the one that asked:

So why are you guys so freaked out by "mysterious" dark matter? Are you feeling the same about neutrinos? I think not..

Now I answer the question by saying, WIMP's may look like neutrinos to you, but to someone that has researched neutrinos for a decade, they look very different to me. Now if they look the same to you, that's fine, but you asked the question.

"Dark matter" is a misnomer. It is not in fact dark, it seems to be transparent. Baryonic matter is not. Even as a dilute gas, it is detectable by observations in EM.

If it's not ionized or not in compact bodies. Ionized hydrogen is quite difficult to detect. One interesting thing is that if you add up all of the baryonic matter that we can account for, it's still much, much less than the amount that we infer is out there from cosmology.

 

[ibysaiyan]

The idea of dark matter comes from mathematical model of gravitational force.
Scientist suggests that there must be matter that we don't see in our universe, maybe between galaxies that exert the gravitational effects that we feel.
The content/percentage of dark matter is chosen to fit and balance all the gravitational effects that has been observed in the universe (expansion of universe, perturbation of orbits, etc).
We predicted the existence of them, but we can't be really sure of what dark matter is.

Some theories such one those who suggest multi-dimensional universe says that dark matter is actually some matters in other universe. String Theory says that only gravitons are able to escape from the membrane of the dimension, hence only gravitational forces are able to penetrate between dimensions. Living in this dimension, we can feel the gravitational effects from other dimension, but we don't see them, that's why we called them Dark Matter.
That's one of the explanation that I liked a lot... although... it's hard to imagine.

Is it ?
I was led to believe that it was more of an observational find rather than resolving a mathematical discrepancy.

We are trying to answer the accountability of mass surrounding the center of galaxies. Basically , Astronomers assumed that mass density (hence radial speed) would decrease as the distance from the galactic radius increases , however, to their surprise it did not and stayed fairly constant.

There are various contenders of DM: Mainly the WIMPs , MACHOS however recent studies highlight towards WIMPs.