跟讀練習: Superfluid Dark Matter - 通過影片學習英語口語

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In this video I want to tell you where I stand on the question of dark matter or modified gravity.

As I said in my two earlier videos, we have a lot of observational evidence that we are misunderstanding something about the way the universe works.

Either there must be a new type of matter, the so -called dark matter, or gravity works differently than Einstein taught us, and this is called modified gravity.

If you ask an astrophysicist, they will tell you that dark matter can explain all the observations

while modified gravity cannot

and therefore dark matter is the better explanation for a long time i thought

that this was a reasonable argument but i no longer think

so before i tell you why i changed my mind i

want you to do a little thought experiment imagine it's the 18th century

and you're daniel bernoulli

and you have just discovered the key equations of fluid dynamics you have tested them back and forth and they work great.

Now you want all your colleagues to check these equations for you.

So you send letters to them and you ask them to please repeat your experiment

and to report back if the equations work.

One after the other they all write back yes the equations work great and you're a genius.

Except for the guy in Alaska who says you're an idiot.

Now you can go and add all kinds of parameters to your equations

and try to explain why water sometimes remains in the shape of a glass.

But that would be entirely insane.

What you want to do instead is to go and look for environmental parameters that explain why water sometimes behaves differently.

Let us now come back to dark matter.

Astrophysicists normally describe dark matter as a fluid.

And I know that can be confusing, but a fluid can be a gas.

Describing dark matter as a fluid works very well for the early universe

and for the cosmic microwave background and for the galaxy clusters.

It does not work so well for the galaxies.

For the galaxies, modified gravity is the much simpler explanation.

So should you go now

and add all kinds of parameters to the equations for dark matter to make them fit the galaxies?

I think that's as insane as trying to explain ice by adding all kinds of parameters to the equations of hydrodynamics.

What we should do instead is look for environmental parameters that separate one behavior from the other.

I think what the data are trying to tell us is

that matter comes into different phases and we have to use different equations for each phase.

The first time I came across this idea was in a 2015 paper by Bereziania and Khoury.

They pointed out that the mathematical structure of modified gravity looks very much like the mathematical structure of a superfluid.

So the two phases of dark matter are not, as in the case of water, a liquid and a solid, but it's a fluid and a superfluid.

What's a superfluid?

A superfluid is fluid that has no internal friction and that has quantum correlations that span over very long distances.

In the case of dark matter, these correlations can span through whole galaxies.

Now, the idea that dark matter may form a superfluid is not new.

What is new here is that the superfluid gives rise to a force which acts on the normal matter.

This new force is the reason why the effects of superfluid dark matter can look like modified gravity.

If you remember what I told you earlier,

you can use this force to calculate the relation between the velocity of a star and the distance from the galactic center.

If you draw this on a graph,

then the force that you get from the normal gravitational pull of the normal matter is not enough to explain the observations.

If you now add the force that comes from the superfluid, it works nicely.

So let me be clear that this additional force is not the gravitational pull from the superfluid.

It's actually a new force that comes from the interaction of the superfluid with the normal matter.

But if you want dark matter to condense to a superfluid,

you need it to be cold and And you need a gravitational potential that is deep enough to create sufficient pressure.

These conditions are not fulfilled in the early universe and they are not fulfilled in the galaxy clusters.

They are fulfilled in the galaxies.

And this explains why modified gravity sometimes works and sometimes doesn't.

Sounds good, huh?

But it's not so simple.

Really what I just told you is some earlier words

and the equations to back it up are not quite there yet the biggest problem is

that it's not very well understood under exactly which conditions dark matter forms a superfluid.

There are also different kinds of particles that can form a superfluid, and it's not clear which of those fits the data best.

Another problem is that it's really not well understood how fluid condenses to a superfluid in a curved spacetime.

That's because the people who normally study superfluids don't have to think about gravity all that much.

If they take it into account at all, it's a vertical gradient in the laboratory.

But you can't describe galaxy formation with that.

Can you experimentally distinguish this type of superfluid dark matter from the normal dark matter?

I believe you can.

This is something that I have been working on with a PhD student in the past year.

His name is Tobias Misteler.

Maybe I will tell you something about this some other time.

The message that I want to get across here is

that I think it's a mistake to regard dark matter and modified gravity as two competing theories,

each of which has to be made to fit all of the data.

To me the data say, the answer is a combination of both.

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跟讀練習: Superfluid Dark Matter - 通過影片學習英語口語

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