Dark Energy: More Evidence

[George Jones]

BBC News - New method 'confirms dark energy'

[toml42]

Interesting, it was nice that the article linked directly to the papers for once!

This must make life very difficult for alternate cosmology models, as photon energy loss clearly can't account for these observations

[JulianO]

I was at a seminar on Wednesday given by one of the people from Portsmouth. The evidence from type 1a supernovae and these "baryon acoustic oscillations" - shock waves left over from the early universe really narrows down the error bars. This makes two independent ways of measuring the effects of dark energy.

While its still possible dark energy and indeed General Relativity might be wrong, or slightly flawed, the wriggle room for them being wrong is narrowing.

Fascinating, but I really want to ask, "so what is Dark Energy" but that apparently really needs a theoretical breakthrough.

[rabbithutch]

Could someone please tell me if the terms Dark Matter and Dark Energy refer to the same phenomenon?

I'm reading does E=mc²(and whyshould we care?) by Brian Cox and Jeff Forshaw. They probably are the brilliant physicists that reputation makes them, but I find their writing skills to be lacking. It frequently takes me multiple readings to suss out their meaning (and I've always been a 99% retention reader with above average IQ and a fair amount of education tho not of the scientific sort). I've not finished it yet; so I don't know if my dark_energy/dark_matter question is addressed therein.

At 69 yoa, I guess it's too late to become a physicist, eh?

[George Jones]

Despite having trained as a scientist, I do not find reading science to be a breeze. Writing science is a lot more difficult than reading science, and I doff my cap to the authors for bravely making the attempt.

Dark matter and dark energy are different things.

Dark matter first. For decades, physicists have observed that stars in galaxies seem to have speeds higher than the escape velocities of galaxies. Galaxies should boil away, but they don't. If, however, there is unseen dark matter in galaxies, then, because of the gravity of this unseen matter, the escape velocities of galaxies are higher than they seem, and the speeds of stars are less than the actual escape velocities.

Now dark energy. Starting in 1998, physicists have observed evidence that the expansion of the universe is speeding up, not slowing down as might be expected. Expansion might be expected to slow down because of the gravitational attraction between all things in the universe. Dark energy is usually the name given to the cause of this acceleration.

[rabbithutch]

Great explanation, George! Thank you!

[JulianO]

Dark matter first. For decades, physicists have observed that stars in galaxies seem to have speeds higher than the escape velocities of galaxies. Galaxies should boil away, but they don't. If, however, there is unseen dark matter in galaxies, then, because of the gravity of this unseen matter, the escape velocities of galaxies are higher than they seem, and the speeds of stars are less than the actual escape velocities.

Just to add to this, one of the first indications there was anything wrong was the rotation of galaxies. If you look at the rotation velocities in the solar system for instance, you find things nearer the sun are going around faster (in terms of their speed through space) than things out near pluto. This is because the orbital speed is based on two factors, the distance and the mass (of the sun in this case). There is a simple mathematical formula that predicts how things will behave. It looks like line A on the graph.

Now when they got around to measuring the speed of stars in nearby galaxies as they rotated around the centre, this didn't match up with theory. It looked like line B. There are two possible explanations for this.

One - the theory of gravitation is wrong.

Two -there is some missing mass, well turns out quite a lot of missing mass needed to fix it. Even if you include dead stars, gas, dust and other postulated things (MACHOS), really you are a long way off getting enough mass to make things right.

People have tinkered with gravity in various ways trying to get it to explain this. Suggesting it works in different ways over very big distances. None of them terribly satisfactory, but they can explain some parts.

Since then, other gravitational effects such as gravitational lensing, (gravity bends light, the more mass the more it is bent) which effectively enables you to weigh galaxies have been used. Yet again, when you weigh them with this technique and then estimate the weight based on the number of stars, they don't match, and by a long way again. Also, often the mass needed to bend the light in the way we see it bent isn't in the right place even if there were enough mass.

So eventually they suggested something called dark matter, which you can only detect gravitationally. It doesn't scatter light, so you can't see it (hence dark!). It tends to clump together in halos under gravitational attraction, but not into dense objects like stars.

Having got this thing called DM, it now explains lots of things. You can simulate how galaxies formed after the big bang, and using just dark matter (after all normal matter is almost negligible). This makes for much simpler models, the DM clumps together in dense spots, the regular matter tends to follow it and form galaxies.

There is even a particle physics theoretical particle that might well fit the bill (the neutralino) although its not been proved to exist yet.

So dark matter fixes a number of problems in the universe, may well have some foundation in theory, and is detectable. Its not 100% nailed down, and the theory may still be proven wrong, but most astronomers accept it now.

As said above, dark energy is the force pushing the universe ever father apart, and at increasing speeds. At present, all they really have for this is a name, the rest of it speculation, but something is definitely speeding up the expansion of the universe, and this is called dark energy. You can calculate how much there is from what it is doing, but thats about all so far.

[Adamski]

I'm going to get me one of those telescopes tomorrow.lol

[tom33pr]

Julian, brilliant - thanks!

Forgive me for a potentially silly question regarding dark matter.

Since the mentioned measurements regarding the velocity of stars in galaxies, and on the other hand those regarding lensing effect failed heavily, while expected to act as our gravity theory would suggest - where the errors the same in both cases?

I mean do we apply some sort of constant to DM - if so , what is it?

Thanks.

[JulianO]

Since the mentioned measurements regarding the velocity of stars in galaxies, and on the other hand those regarding lensing effect failed heavily, while expected to act as our gravity theory would suggest - where the errors the same in both cases?

Well - not really. In some ways yes, in that the results only made sense if there was a lot more mass there than we could see. In the case of galaxy rotation curves, its not only the amount, but where it is too.

The curves above assume that most of the mass is in the centre of the galaxy, and it thins out as you get further out. This is broadly in line with observed star density in galaxies. The actual rotation curves behave as if there is more mass, but also spread out, so it doesn't diminish much with distance. So in this case, the dark matter is a sort of diffuse halo that encapsulates the entire galaxy and then some.

So just having lots more mass in the centre wouldn't give the observed results.

With lensing, its more the quantity of stuff.

You can think of it like a conventional optical lens, but a rather misshapen one, and you look through it at an object.

What you see is a distorted image, but if you were clever, and you knew roughly what the image should look like, you could work out what sort of lens distortion is required to give you what you see. This would tell you how thick the glass was across the lens. Its similar with gravity, but replace glass with mass. So looking at a distorted view, its possible to work out where the mass is, and how much there is to give the distortion. In some cases this shows a lot of mass where there is nothing very obvious.

[tom33pr]

Julian, thanks a lot for that - very well explained. I wish all difficult issues were worded this comprehensive way.

[rabbithutch]

Thank you, Julian!

I'll re-read that several times again before I even begin to understand it even though you've written quite well. It's the concepts this old brain is struggling to master.