Over 1.5 million copies sold
Books by Richard Dawkins
The Oxford Book of Modern Science Writing
by Richard Dawkins
The Selfish Gene
by Richard Dawkins
The Blind Watchmaker
by Richard Dawkins
The Ancestor's Tale
by Richard Dawkins
Unweaving the Rainbow
by Richard Dawkins
Climbing Mount Improbable
by Richard Dawkins
Print
Cosmic crash unmasks dark matter
by BBC NEWS
Thanks to Mark8 for the link.
http://news.bbc.co.uk/2/hi/science/nature/7587090.stm
Cosmic crash unmasks dark matter
By Paul Rincon
Striking evidence has been found for the enigmatic "stuff" called dark matter which makes up 23% of the Universe, yet is invisible to our eyes.
The results come from astronomical observations of a titanic collision between two clusters of galaxies 5.7 billion light-years away.
Astronomers detected the dark matter because it separated from the normal matter during the cosmic smash-up.
The research team are to publish their findings in the Astrophysical Journal.
They used the Hubble and Chandra space telescopes to study the object MACSJ0025.4-1222 - formed after an incredibly energetic collision between two large galaxy clusters.
Each of these large clusters contains about a quadrillion times the mass of our Sun.
A technique known as gravitational lensing was used to map the dark matter with Hubble.
If an observer looks at a distant galaxy and some dark matter lies in between, the light from that galaxy gets distorted.
It looks as if it is being seen through lots of little lenses. And each of these lenses represents a piece of dark matter.
Astronomers used the Chandra X-ray telescope to map ordinary matter in the merging clusters, mostly in the form of hot gas, which glows brightly in X-rays.
As the two clusters that formed MACSJ0025 merged at speeds of millions of kilometres per hour, hot gas in the two clusters collided and slowed down.
However, the dark matter kept on going, passing right through the smash-up.
Speeding bullet
This phenomenon has been seen before, in a structure called the Bullet Cluster - which also formed after the collision of two large galaxy clusters. The Bullet Cluster lies closer to Earth, at a distance of 3.4 billion light-years.
"It puts to rest all the worries that the Bullet Cluster was an anomalous case. We have gone out and found another one," co-author Richard Massey, from the Royal Observatory Edinburgh, told BBC News.
The study sheds light on the properties of dark matter.
The fact that dark matter does not slow down in the collision supports a view that dark matter particles interact with each other only very weakly or not at all (when one excludes their gravitational interaction).
"Dark matter makes up five times more matter in the Universe than ordinary matter," said co-author Marusa Bradac, from the University of California at Santa Barbara (UCSB).
"This study confirms that we are dealing with a very different kind of matter, unlike the matter that we are made of. And we're able to study it in a very powerful collision of two clusters of galaxies."
Larger sample
The latest astronomical observations suggest that dark matter makes up some 23% of the Universe. Ordinary matter - such as the galaxies, gas, stars and planets - makes up just 4%.
The remaining 73% is made up of another mysterious quantity; dark energy, which is responsible for speeding up the expansion of the cosmos.
According to one model, dark matter may be comprised of exotic sub-atomic "stuff" known as Weakly Interacting Massive Particles (WIMPS).
Others hold that the dark substance consists of everyday matter, rather than some elusive sub-atomic particle. However, this ordinary matter, referred to as Massive Astrophysical Compact Halo Objects (MACHOS), happens to radiate little or no light.
A powerful physics experiment, the Large Hadron Collider, which is currently under construction on the French-Swiss border, could shed further light on this question after it begins operating later this year.
Dr Massey said his team had found other candidates for colliding clusters.
"Ideally, we don't want just one or two, we want lots of these things to really study them statistically," he explained.
"Then we either use the whole lot, or pick out one 'golden bullet' which will provide the best constraints on what dark matter is."
The Hubble Space Telescope failed just after the team had taken their image of MACSJ0025, so they have not yet been able to study these other candidates.
Dr Massey said the astronomers hope to do this after the next Hubble servicing mission with the space shuttle, which is due to launch in October 2008.
http://news.bbc.co.uk/2/hi/science/nature/7587090.stm
Cosmic crash unmasks dark matter
By Paul Rincon
Striking evidence has been found for the enigmatic "stuff" called dark matter which makes up 23% of the Universe, yet is invisible to our eyes.
The results come from astronomical observations of a titanic collision between two clusters of galaxies 5.7 billion light-years away.
Astronomers detected the dark matter because it separated from the normal matter during the cosmic smash-up.
The research team are to publish their findings in the Astrophysical Journal.
They used the Hubble and Chandra space telescopes to study the object MACSJ0025.4-1222 - formed after an incredibly energetic collision between two large galaxy clusters.
Each of these large clusters contains about a quadrillion times the mass of our Sun.
A technique known as gravitational lensing was used to map the dark matter with Hubble.
If an observer looks at a distant galaxy and some dark matter lies in between, the light from that galaxy gets distorted.
It looks as if it is being seen through lots of little lenses. And each of these lenses represents a piece of dark matter.
Astronomers used the Chandra X-ray telescope to map ordinary matter in the merging clusters, mostly in the form of hot gas, which glows brightly in X-rays.
As the two clusters that formed MACSJ0025 merged at speeds of millions of kilometres per hour, hot gas in the two clusters collided and slowed down.
However, the dark matter kept on going, passing right through the smash-up.
Speeding bullet
This phenomenon has been seen before, in a structure called the Bullet Cluster - which also formed after the collision of two large galaxy clusters. The Bullet Cluster lies closer to Earth, at a distance of 3.4 billion light-years.
"It puts to rest all the worries that the Bullet Cluster was an anomalous case. We have gone out and found another one," co-author Richard Massey, from the Royal Observatory Edinburgh, told BBC News.
The study sheds light on the properties of dark matter.
The fact that dark matter does not slow down in the collision supports a view that dark matter particles interact with each other only very weakly or not at all (when one excludes their gravitational interaction).
"Dark matter makes up five times more matter in the Universe than ordinary matter," said co-author Marusa Bradac, from the University of California at Santa Barbara (UCSB).
"This study confirms that we are dealing with a very different kind of matter, unlike the matter that we are made of. And we're able to study it in a very powerful collision of two clusters of galaxies."
Larger sample
The latest astronomical observations suggest that dark matter makes up some 23% of the Universe. Ordinary matter - such as the galaxies, gas, stars and planets - makes up just 4%.
The remaining 73% is made up of another mysterious quantity; dark energy, which is responsible for speeding up the expansion of the cosmos.
According to one model, dark matter may be comprised of exotic sub-atomic "stuff" known as Weakly Interacting Massive Particles (WIMPS).
Others hold that the dark substance consists of everyday matter, rather than some elusive sub-atomic particle. However, this ordinary matter, referred to as Massive Astrophysical Compact Halo Objects (MACHOS), happens to radiate little or no light.
A powerful physics experiment, the Large Hadron Collider, which is currently under construction on the French-Swiss border, could shed further light on this question after it begins operating later this year.
Dr Massey said his team had found other candidates for colliding clusters.
"Ideally, we don't want just one or two, we want lots of these things to really study them statistically," he explained.
"Then we either use the whole lot, or pick out one 'golden bullet' which will provide the best constraints on what dark matter is."
The Hubble Space Telescope failed just after the team had taken their image of MACSJ0025, so they have not yet been able to study these other candidates.
Dr Massey said the astronomers hope to do this after the next Hubble servicing mission with the space shuttle, which is due to launch in October 2008.
Add to del.icio.us
Posted: Monday, September 1, 2008 | PermalinkComments 1 - 40 of 40 |
Reload Comments | Back to Top | Page Numbers2. Comment #240754 by stephenray on September 1, 2008 at 2:25 am
It's rather sloppy to say that dark matter is 'invisible to our eyes'. Half the phenomena in the universe is invisible to our eyes - radio waves, infra red, neutrinos, and so on.The Beeb ain't what it used to be.
Other Comments by stephenray
3. Comment #240766 by Oystein Elgaroy on September 1, 2008 at 2:52 am
Regarding WIMPS vs MACHOS the odds are in favour of WIMPS. As mentioned in the article, MACHOS are built from ordinary baryonic matter. Models and observations of the synthesis of light elements in the early universe , and the temperature variations in the cosmic microwave background tell us that the contribution of baryons to the total energy density is only about four percent. At the same time both the microwave background and studies of how structure formed in the universe show that the total contribution from matter to the energy budget is about 25 percent. So most of the dark matter has to be non-baryonic, and the favourite candidate is one of the heavy, weakly interacting particles predicted to exist in extensions of the Standard Model of particle physics. Other Comments by Oystein Elgaroy
4. Comment #240767 by dvespertilio on September 1, 2008 at 2:53 am
stephenray: "phenomena" is a plural noun, the singular is "phenomenon," as in "Half the phenomena in the universe ARE invisible to our eyes, etc"Other Comments by dvespertilio
5. Comment #240772 by ficklefiend on September 1, 2008 at 3:02 am
I suspect physicists get away with their humourous acronyms because they are the only ones who actually understand what they mean. Other Comments by ficklefiend
6. Comment #240780 by decius on September 1, 2008 at 3:12 am
The fact that dark matter does not slow down in the collision
How could they determine this?
Other Comments by decius
7. Comment #240787 by amalthea on September 1, 2008 at 3:25 am
They have a sense of humour alright:
http://www.youtube.com/watch?v=j50ZssEojtM&eurl=http://www.boingboing.net/2008/07/31/large-hadron-rap-bes.html
:)
Other Comments by amalthea
8. Comment #240788 by Quetzalcoatl on September 1, 2008 at 3:31 am
Decius-
How could they determine this?
Here is some more detail. By pinpointing the dark matter's location they were able to calculate the speed it must have been travelling at to reach that point, and therefore deduce that it had not been slowed down by the collision.
Other Comments by Quetzalcoatl
9. Comment #240790 by Oystein Elgaroy on September 1, 2008 at 3:43 am
Comment #240780 by deciusThey can't, this is part of the assumption they are testing here. The observable matter in galaxy clusters is dominated by hot gas emitting X-rays, but most of the matter is expected to be dark matter. Picture the two galaxy clusters as two spherical blobs of matter. When the collide, the gas components will interact with each other and there will be a concentration of it in the collision region. The dark matter in the two blobs will just pass freely past each other. After quite some time what you will be left with is two intersection blobs of matter where the hot gas is concentrated in a small region of overlap whereas most of the dark matter will be found outside of this region. X-ray observations can tell you where the gas is, and observations of how the two clusters distort the images of background galaxies allow you to map out the gravitational field of the cluster and hence where most of the mass is. The fact that the observations agree with the simple picture above suggests that the dark matter indeed is some kind of weakly interaction particle.
Other Comments by Oystein Elgaroy
10. Comment #240791 by decius on September 1, 2008 at 3:47 am
Many thanks, Quetz and Oystein. Other Comments by decius
11. Comment #240811 by Steven Mading on September 1, 2008 at 4:53 am
4. Comment #240767 by dvespertilio on September 1, 2008 at 2:53 am
stephenray: "phenomena" is a plural noun, the singular is "phenomenon," as in "Half the phenomena in the universe ARE invisible to our eyes, etc"
If you're going to get petty about grammar, be more careful. The noun that the verb needed to agree with was not the word "phenomena". It was the entire phrase "half the phenomena". The relevant noun was actually "half" not "phenomena". (American usage will treat it as singular, British will treat it as plural. Both are correct. In American usage the phrase "half the somethings" describes a set, which puts it in singular context (because you're talking about the collected set as one group). In British usage the tendency is not to treat it like a set and still keep it plural.
Is any of this relevant? Of course not, but you're the one who wanted to harp on a minor grammar point.
Other Comments by Steven Mading
12. Comment #240813 by George Lennan on September 1, 2008 at 5:00 am
Strikes me that if dark matter is 73 % and ordinary matter just 27 % we should be calling 'dark matter 'ordinary matter' and that it's us who are made of the weird stuff! Other Comments by George Lennan
13. Comment #240820 by Ian Bamlett on September 1, 2008 at 5:19 am
How do other dimensions fit into this? We are told are we not there is a fourth, fifth, six and maybe more dimension? Does dark matter have anything to do with this - maybe it's all we can dimly perceive of these other dimensions. Or are they totally unrelated?That would make for some cool sci fi - someone has probably already done it I bet.
Other Comments by Ian Bamlett
14. Comment #240823 by Oystein Elgaroy on September 1, 2008 at 5:30 am
Comment #240820 by Ian Bamlett There is no necessary connection between dark matter and extra dimensions. All we need is a new, weakly interacting particle. There are some more inventive models where the effects we ascribe to dark matter are caused by our universe interacting with another "dark universe" via a fifth dimension. They are fun, but my money is on the dark matter being one of the WIMPs found in supersymmetric extensions of the Standard Model.
What is really exciting about dark matter is that it is a clear signature of new physics. To explain the observations we must either introduce a new particle, or we must modify general relativity.
Other Comments by Oystein Elgaroy
15. Comment #240826 by NewEnglandBob on September 1, 2008 at 5:38 am
Article stated:The latest astronomical observations suggest that dark matter makes up some 23% of the Universe. Ordinary matter - such as the galaxies, gas, stars and planets - makes up just 4%.
The remaining 73% is made up of another mysterious quantity; dark energy, which is responsible for speeding up the expansion of the cosmos.
12. Comment #240813 by George Lennan misstated these.
Strikes me that if dark matter is 73 % and ordinary matter just 27 % we should be calling 'dark matter 'ordinary matter' and that it's us who are made of the weird stuff!
Read more carefully.
Other Comments by NewEnglandBob
16. Comment #240847 by Sargeist on September 1, 2008 at 7:05 am
For those people here who are interested in the cutting edge of physics, there is a very informative video about the LHC on YouTube:http://www.youtube.com/watch?v=j50ZssEojtM
Other Comments by Sargeist
17. Comment #240851 by LBraschi on September 1, 2008 at 7:09 am
Interesting article, although I hate it when it is formatted in a single sentence per line. As if making paragraphs longer than a single sentence would be too difficult for people to read. Talk about dumbing down science.
Other Comments by LBraschi
18. Comment #240865 by Ian Bamlett on September 1, 2008 at 7:47 am
RE: Comment #240823 by Oystein Elgaroy
Thanks Oystein! So cool to have a professor of astrophysics on the site who will kindly answer a question or two.
Gotta love this site.
:-)
Other Comments by Ian Bamlett
19. Comment #240870 by Rational_G on September 1, 2008 at 7:56 am
Some more info from the Chandra site:http://chandra.harvard.edu/photo/2008/macs/
Other Comments by Rational_G
20. Comment #240875 by GBile on September 1, 2008 at 8:03 am
YECs,Read this article, then either be very much ashamed or crawl back in your holes and never show your faces again.
Other Comments by GBile
21. Comment #240879 by Apathy personified on September 1, 2008 at 8:15 am
Oystein,What are the leading theories on dark energy? I haven't actually heard or read about any (although that is probably more a reflection on my own ignorance)
I really think that cosmology is shining a bright light on the incompleteness of the standard model - let's hope that the LHC together with the cosmological observations give us a new 'knowledge revolution' in physics, something that is sorely needed.
Other Comments by Apathy personified
22. Comment #240883 by Rational_G on September 1, 2008 at 8:22 am
There's a chance the LHC may detect dark matter -from physicist Sean Carroll:
http://cosmicvariance.com/2008/08/04/what-will-the-lhc-find/
Other Comments by Rational_G
23. Comment #240933 by Oystein Elgaroy on September 1, 2008 at 10:08 am
Sorry for the Joe Morreale-length of this post. Comment #240865 by Ian Bamlett
Thanks! I consider it part of my job. It is the only way I can excuse all the time I spend on this site
Comment #240879 by Apathy personified
What are the leading theories on dark energy? I haven't actually heard or read about any (although that is probably more a reflection on my own ignorance)
All observations we have are consistent with the dark energy being Einstein's good old cosmological constant (quantum vacuum energy). Before the first observations indicating cosmic acceleration appeared in 1998, it was common to assume that the cosmological constant was vanishingly small. However, there never were any good theoretical arguments for this. So in this sense the cosmological constant is the best solution of the dark energy problem. It is simple, it is already there, and it is consistent with all observations. So why all the fuss about dark energy?
As you probably know already there are a couple of problems with the cosmological constant as dark energy. The most dramatic one is its smallness. The quantum vacuum energy receives contributions from zero-point fluctuations of all quantum fields in the Standard Model, and the "natural" value is either set by the Planck energy or by the energy scale where supersymmetry is broken, depending on your convictions. In either case the value of the vacuum energy required to explain the observations is at least sixty orders of magnitude larger than the natural value. This is a bit of a puzzle, to say the least.
Faced with this problem one can go in two fundamentally different directions. The first is to invoke the anthropic principle and say that the cosmological constant has to have a value consistent with our existence. If it is more than roughly a hundred times the observed value, it would have started dominating the expansion of the universe before galaxies had formed, and we would never have been here. Of course for such an explanation to really work you need to assume the existence of a multiverse and that the quantum vacuum energy is a random variable that varies between the different island universes.
Another possibility is to hope that we one day will find some mechanism or symmetry principle that will make the cosmological constant vanish exactly. No one knows what this could be, but let us suppose that this turns out to be the case. In this case we will need another explanation for dark energy. There is no known field in the Standard Model that can do the job, so we will have to invent something. Two main approaches are common: introduce a new scalar field or modify general relativity.
A scalar field is similar to a cosmological constant expect that it varies with time. Extensions of the Standard Model typically contain several new scalar fields so the hope is that one of them can serve as dark energy. The problem with this approach is that the wavelength of the field has to be of the order of the observable universe, which implies that the field must be very light. If it couples to matter this would give rise to a new "fifth force", and this is more or less ruled out by experiments. These problems can be avoided by a so-called chameleon field that couples to matter according to the density of its surroundings. This mechanism actually gives rise to effects that should be observable in table-top experiments with lasers and vacuum chambers, so these models could be ruled out or confirmed within a couple of years.
General relativity can be modified in several ways. The most obvious way is to tinker with Einstein's field equations and make them more complicated. To my mind this is not a very attractive approach. You end up with horrendous equations, and in many of these models you have embarrassing problems like not getting special relativity as a special solution in the limit of vanishing density.
Another way general relativity can be modified is by extra dimensions. For this to be relevant for dark energy the extra dimensions have to be large. The idea is that the universe started to expand differently once it's characteristic size (set by the Hubble parameter) became comparable to the size of the extra dimensions. Some of these models are quite cute, but they are not without their problems. They may not make sense as quantum theories and some of them have problems with explaining all the observations.
One final possibility I should mention is to abolish dark energy completely. In a universe with large-scale homogeneity the distance-redshift relationship derived from distant supernovae imply that the expansion rate is increasing. But if the universe is inhomogeneous this interpretation is no longer valid. It is possible to explain the supernova observations in a model where we live near the centre of a large bubble where the density is lower than in the surrounding space. However, this model suffers from the obvious problem that it places us at the center of the observable universe.
My personal opinion is that all the evidence points in the direction of a cosmological constant. No observation suggests that something more complicated is involved, and it is already there in our models. My favourite alternative is the chamelon field since it is actually possible to test in table-top experiments. However, I wouldn't be surprised if I am wrong.
Other Comments by Oystein Elgaroy
24. Comment #240949 by George Lennan on September 1, 2008 at 10:31 am
Pedant! There's loads more dark matter than the sort we're made of, so my highly insightful and consciousness-raising point stands, despite the evident lack of rigour in my reading of scientific texts (I lost my job as a space rat scientist because of this too - I'm a barman now! Ah well.) Other Comments by George Lennan
25. Comment #240971 by Naturalist1 on September 1, 2008 at 11:04 am
My particular(no pun intended)thanks to Amalthea in post #7 for the excellent link about the LHC. I live in the Waterloo area of Ontario, Canada so regularly attend the public lectures at The Perimeter Institute of Theoretical Physics. This little video makes cosmology understandable to virtually anyone...it is just a hoot. It so cleaverly explains what LHC is all about and why it is so important.
Thank you Amalthea.
And to Oystein Elgaroy, a very insightful posting. You conclude:
"My personal opinion is that all the evidence points in the direction of a cosmological constant. No observation suggests that something more complicated is involved, and it is already there in our models."
I agree...that which, at the time Einstien called his greatest blunder may in fact have been his greatest genius insight...LHC may let us know.
Other Comments by Naturalist1
26. Comment #240974 by Oystein Elgaroy on September 1, 2008 at 11:09 am
Comment #240971 by Naturalist1 I agree...that which, at the time Einstien called his greatest blunder may in fact have been his greatest genius insight...LHC may let us know.
As Rocky Kolb (famous Fermilab cosmologist) remarked in a lecture: "If Einstein had stuck to his idea, he would have been famous!"
Other Comments by Oystein Elgaroy
27. Comment #240983 by Apathy personified on September 1, 2008 at 11:21 am
Thanks for the indepth reply, Oystein, exactly what i need to get my brain into gear, ready for october.I'm not a fan of the anthropic ideas - they never seem to give a satisfactory explanation of 'why?' it's like it is, they just give a 'what?' as an answer, though i have to add again - i have only a basic grasp of the vague ideas and no real technical knowledge.
I definately would like the chamelon field, i think it has a 'neater' tie-in with the rest of the standard model and it would remove another experimental constant (or can the cosmological constant be derived?)
As we can't observe it from Earth, can a fifth force that is incredibly long range (galactic range) be discounted? Although i don't know how such a force would work or if such a force could explain the origin of dark energy and dark matter, or the phenomena that we call dark energy and dark matter.
Other Comments by Apathy personified
28. Comment #240990 by Oystein Elgaroy on September 1, 2008 at 11:38 am
Apathy, The cosmological constant can, sadly, not be derived. There was for some time a hope that it could be derived in string theory, but what we seem to be left with is the string landscape and the anthropic principle.
We cannot discount a long-range fifth force if it couples weakly to matter on small scales (solar system and below). But then you would have a new fine-tuning problem: why should this scalar field couple so weakly to matter? The chameleon mechanism solves this problem by making the coupling depend on the local matter density.
Other Comments by Oystein Elgaroy
29. Comment #241014 by Apathy personified on September 1, 2008 at 2:33 pm
Oystein,Thanks again for the reply.
I remember reading that in the beginning, the hope was that string theory would reduce the number of experimental constants needed and would unify all forces - shame that instead string theory seems to have greatly added to the number of constants and is not near any quantum gravity solution (as far as i know), ah well such is life.
So any long range fifth force would necessarily have to have the chameleon mechanism, or something similar, to be consistant - hmmm, kinda back to square one then.
Does this come down to Einstein being wrong about the cosmological constant (when he said it didn't exist), or there being a small error in GR, that is only noticable on very large distances?
Having typed that, it does look like i'm guilty of a setting up a false dichotomy. I apologise to Einstein.
Other Comments by Apathy personified
30. Comment #241045 by debacles on September 1, 2008 at 4:11 pm
whats this dark matter not interacting?
i assume it interacts a lot less but still must interact... didn't we find out that dark matter makes up most of the matter in galaxies?
or is it so strongly affected by normal matter that it is bound to group itself?
help oystein
Other Comments by debacles
31. Comment #241109 by KRKBAB on September 1, 2008 at 7:38 pm
"The Bullet Cluster lies closer to Earth, at a distance of 3.4 billion light years away. It's right around the corner from us, as opposed to the two colliding clusters this articale is mainly about which are 5.7 billion light years away. Amazing stuff to a simpleton like me. "Who need's the supernatural, the natural is super enough!". Carl SaganOther Comments by KRKBAB
32. Comment #241147 by JDAM on September 1, 2008 at 9:42 pm
Thanks to Amalthea and the link she provided (see comment #7) I have this rap completely stuck in my head! I think I have watched it now about 15 times and it just cracks me up every time! I also have a much more clear understanding of just exactly what the LHC will be looking for. Just think of the success the powers that be could have teaching physics like this! You have got to watch this...it is just too much fun!! Profound thanks to Oystein also for his generous explanatory contributions.Other Comments by JDAM
33. Comment #241163 by Oystein Elgaroy on September 1, 2008 at 10:45 pm
Comment #241045 by debacles You are right, the dark matter interacts. Since it emits no detectable electromagnetic radiation (that's why it is dark) it must be some kind of neutral particle with no electromagnetic interactions. If it is a WIMP (Weakly Interacting Massive Particle) it couples to other particles with the strength of the weak nuclear force (the one responsible for e.g. beta decay of nuclei). "Massive" here means a few hundred times the mass of the proton. One can show that a particle with these properties will be produced in the early universe and end up giving a contribution to the mass density of the universe of just about the right magnitude to make up the dark matter.
The dark matter interacts through gravitation as well, of course, just like any other particle. Otherwise it would, as you rightly remark, have been pretty useless.
Other Comments by Oystein Elgaroy
34. Comment #241166 by Oystein Elgaroy on September 1, 2008 at 10:53 pm
Comment #241014 by Apathy personified Does this come down to Einstein being wrong about the cosmological constant (when he said it didn't exist), or there being a small error in GR, that is only noticable on very large distances?
We don't know. As I said in an earlier post, all observations are consistent with a cosmological constant. The traditional wisdom in the field has been that you would expect GR to be modified at small distances (because of quantum gravity) but not at large distances. Still, some have attempted to modify the large-distance behaviour of GR to explain the observations, but the theories you end up with are ugly. In addition they do not explain why the cosmological constant should be set to zero, they just assume it.
Other Comments by Oystein Elgaroy
35. Comment #241169 by LeeC on September 1, 2008 at 11:45 pm
Excellent...I just this week write up on my blog that the bullett cluster is the best evidence yet for dark matter... then another cluster just comes along.
Love it
Lee
EDIT:
If anyone is interested - I did try to outline the physics going on in the bullet cluster and explain how it proves dark matter in simple terms here
I also provided links to a video showing what science thinks happened and other links as well since I am just a nobody - I just like science.
EDIT on the EDIT
Actually I've just read Oystein Elgaroy comments and it has made my attempt a bit useless. Oh well - I tried :) Just hope I was close.
Other Comments by LeeC
36. Comment #241585 by quantum_flux on September 2, 2008 at 4:32 pm
Cool! Somebody is going to get to the bottom of this dark matter and dark energy thing someday, and I hope it means the possibility of faster than light travel and perpetual motion (er, at least by today's standards it would be considered voodoo and Maxwell's Demon). Other Comments by quantum_flux
37. Comment #241586 by quantum_flux on September 2, 2008 at 4:34 pm
Perhaps the universe is a closed lava lamp that interacts with dark matter and dark energy and spacetime in ways we are only beginning to imagine. Other Comments by quantum_flux
38. Comment #241806 by adrianpatrick on September 3, 2008 at 5:15 am
[Comment #240813 by George Lennan - Strikes me that if dark matter is 73 % and ordinary matter just 27 % we should be calling 'dark matter 'ordinary matter' and that it's us who are made of the weird stuff!]
Okay so the numbers aren't totally accurate - but this is a very good point I think!
Other Comments by adrianpatrick
39. Comment #242484 by Rational_Skeptic on September 3, 2008 at 9:41 pm
Reading articles like this always sends me into "what if" territory:What if I had chosen my first love - astronomy, astrophysics, cosmology - over the social sciences?
Is it too late for a reasonably bright woman in her early forties to change careers?
Fascinating stuff.
Other Comments by Rational_Skeptic
40. Comment #242722 by bluebird on September 4, 2008 at 11:42 am
Thanks for this article; I presume this is why APOD reposted the 'Bullet-Cluster' recently http://antwrp.gsfc.nasa.gov/apod/ap080823.html
Good news about the LHC scheduled to officially operate this fall; *Anticipation, is keeping me waiting!*
Speaking of Carl Sagan, this bit of news pleases me very much: http://jpl.nasa.gov/news/news.cfm?release=2008-170
GLAST was catchy, alas they renamed it. Anyway, this is so neat-- http://www.n2yo.com/?s=33053
Other Comments by bluebird
Upcoming Events:
Mailing List
Enter your email:
DVDs
'The Enemies of Reason: The Uncut Interviews' DVD
'Voices of Reason' DVD
'Richard Dawkins: Appearances & Events 2007-2008' DVD
'Enemies of Reason' DVD
'Root of All Evil?' DVD
'Root of All Evil? The Uncut Interviews' DVD
'Growing Up in the Universe' DVD
'Voices of Science' DVD
'The Four Horsemen' DVD
'Break the Science Barrier' DVD
1. Comment #240751 by theantitheist on September 1, 2008 at 2:21 am
Who says Astronomers don't have a sense of humor?
To come up with MACHOS they must have spent a lot of time smoking some good jamaican matter.
Brilliant stuff all of this anyway!
Galaxies go BOOM!!
Other Comments by theantitheist