Wednesday, October 05, 2011

Dark Energy - the basics

The Nobel Prize for Physics was awarded yesterday for the discovery of dark energy in 1998.

What is dark energy? Do we really need to just accept that it's complicated and freaky, unless we're boffins?

I don't think so.

It's an apparently constant 'energy' per unit volume of space, which causes space to expand.

In 1915, Einstein developed a theory of gravity, out of essentially nothing more than the assumption that the laws of physics in free-fall are the same as those without gravity.

One of his very clear conclusions was that dark energy – which he called a "cosmological constant" – could be a physical aspect of gravity. It emerges naturally from following through the logic from that one starting point.

The question of how to follow the logic is the tricky bit... but unless you're masochistic or deeply suspicious or fantastically curious and patient, it's ok to just think of it as something that's been accepted as a logical implication for nearly a century, and go with it.

So it's been there in the standard modern theory of gravity since the very beginning, although there was no evidence that it was anything other than zero until 1998. It's not a new thing - it's just a part of the nature of the force of gravity.

It's the part of gravity that causes space to expand so that very distant things accelerate away from each other.

I think calling it 'energy' and 'dark' makes it sound freaky and mysterious and new and unknown.

What's new is that it's been measured. Nobody expected it not to be zero, but it's not; and now we can't just pretend it's not there any more. And this is what the winners of the Prize – Perlmutter, Riess and Schmidt – with the help of many, many others, have given to the world.

It's not some kind of bolt-on to the laws of physics to explain something nobody understands – not in any way. It's a sophisticated measurement of something surprisingly simple, and what's more it's an interpretation that's been verified by many other independent observations of what's out there.

And yes, lots of research needs to be done to check that it's not this kind of field or that kind of modification or that it's doing this or that crazy thing, which is very important and great fun for the scientific community... but as it stands, there's no evidence for anything beyond good old gravity, doing its good old Einsteinian thing.

And if it turns out that it is as simple as that, then it means the fate of the universe is that clusters of galaxies will separate over time until they're no longer visible to each other.

Within clusters of galaxies, which is where we live, dark energy doesn't really do anything at all. (Apart from handily ensuring that the entire rest of the universe won't come falling in and crush us at some point in the future!)

I (try to) study this stuff, so it's fascinating to me. The details of the logic of the theories can be daunting, but I like to think that the real substance of ideas like this are accessible to anyone. But perhaps this belief just helps me feel less isolated from those not mad enough to dive into it all in detail.

If you kinda knew all that, and have been trying to come to terms with how it all fits together and the various questions and apparent paradoxes it throws up, the excellent Sean Carroll has provided a very clear and detailed FAQ.


maeda said...

thank you

Yusuf said...

Greetings Bob. Thank you for your wonderful blog. I arrived here looking for some refutations of The Renascence Project and their unified harmonics theory. I wanted to ask some basic questions, but it seems like those discussions have been closed. With regards to this theory of dark matter - it just seems to me like it is something imagined by the scientific community as sort of a filler to explain and give reasoning to occurrences and phenomena that is otherwise not understood. You have been through many discussions with others regarding the defense of science from conspiracy theories and so forth. But, I'd like to argue that the whole approach of looking at science (and the community surrounding it) as some sort of separate entity is very misleading. I'll leave it at that for now because I don't know if you really even read or comment on this blog anymore. If you do respond, then I hope to pursue and actual discussion. Thanks in advance

Bob said...

Hi Yusuf. I'll open the discussions again over there.

If you want to say that dark matter is imagined to explain things we don't otherwise understand, then you could say exactly the same about gravity itself. If you're honest, you don't know there's such a thing as the force of gravity - what you know is that most things seem to fall downwards and you don't know why.

So is the force of gravity imagined? In a way, it is. But we think of the force of gravity is scientifically a good idea. The reason it's good is that it can be used to describe, explain, communicate, and most importantly to reliably predict what will happen to a huge variety of things in an extremely huge variety of situations, from tiny bubbles to galaxies.

If you accept the force of gravity as a working idea (no need to 'believe' in anything) then that theory will enable you to do all kinds of things - fly planes, launch satellites, construct tall buildings. If you prefer to see it as a mystery, then you won't. Simple as that.

When you have an idea that faithfully reflects some aspect of nature to such precision in so many ways, you have to admit there's some insight into reality there.

The notion of dark energy has those same features. If you're a cosmologist or large-scale astrophysicist, then if you choose to accept dark energy as a working idea, you will be enabled to explore a great deal of the distant or early cosmos - much is opened up to you. If you prefer to see it all as a mystery, then you won't. Simple as that. It's a powerful theory. You probably don't feel that in the way you do for the force of gravity, because you don't have to work as a cosmologist.

Saying it's invented is a bit like saying to an aircraft manufacturer that engines have to run on chicken poo. You can believe that if you like - it doesn't really matter, because you don't have to build aircraft engines. If you did, you'd soon realise that it was false.

I hope that makes some sense.

"I'd like to argue that the whole approach of looking at science (and the community surrounding it) as some sort of separate entity is very misleading" - well, I haven't a clue what this is supposed to mean. Of course that would be misleading. No scientist would look at science in that way.

The point of science is to honestly and carefully explore the world, precisely as it is, on its own terms, free from the crippling preconceptions, opinions and tendency to invent stories that all humans have. The subject of science is everything, and science is open to everyone.

Anyone who is prepared to lay aside their opinions and preconceptions about how the world should be can embark on the process of learning whatever science they choose. There are no boundaries.

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