[MUSIC PLAYING] What do the first stars in the universe, dark matter, and superior siege engines have in common?

It's that I'm about to blow your mind, talking about all three.

[MUSIC PLAYING] Sometimes, space news sneaks by without getting much attention.

Not everything wows, like gravitational waves or space-faring sports cars.

That's the case with the recent discovery of the earliest stars in the universe.

In a nature paper published just a few weeks ago, Judd Bowman and collaborators, report a signal from the very first stars to form in our universe.

The same result also hints at brand new physics that may help us explain the nature of dark matter.

This result flew under the radar, in part, because it's a subtle and clever result that requires a bit of interpretation.

Today, we're doing a Space Time Journal Club to explain this discovery.

We'll follow that, with the solution to our recent Trebuchet Challenge question.

[MUSIC PLAYING] So the very early universe was full of hydrogen gas and light.

That light was the leftover heat glow from before those first hydrogen atoms formed.

This is the cosmic microwave background radiation, or CMB.

It's the oldest light that we can see, and we explained it in detail in a previous episode.

We can also, try to see the light signature from that very early hydrogen gas.

We do that by looking for a very particular type of photon-- the one that is released or absorbed, when the ground state electron and hydrogen flips its spin direction.

That photon has a wavelength of 21 centimeters, which is radio light.

In the early universe, the rate of hydrogen spin flip was in equilibrium with the CMB, meaning that for every CMB photon that was absorbed by the spin flip, another one was emitted.

We say that the electron spin temperature was coupled to the CMB temperature.

The upshot is, that the earliest 21 centimeter radiation is hopelessly mixed with the CMB, which means, it's impossible to distinguish.

At least to start with.

Before long, some of that early hydrogen gas collapsed to form the very first stars, long before the first galaxies formed.

The ultraviolet light from those stars shifted the equilibrium so that the electron spin temperature became connected to the temperature of the gas, instead of the CMB.

That change in equilibrium meant the gas was suddenly absorbing more 21 centimeter photons, than it was emitting.

After a while, the first black holes formed, and started to spew out x-rays, as they gobbled up hydrogen.

This heated the gas and eventually, became too hot to emit, or absorb, 21 centimeter of photons at all.

[MUSIC PLAYING] The TLDR is that there should have been this brief period of time when the universe was eating up 21 centimeter photons from the CMB.

That should look like a dip in the CMB spectrum.

Now, remember also, that the universe was expanding back then, just like it is now.

Absorption at 21 centimeters would now look like absorption at a much longer wavelength.

In fact, there should be this broad dip at a range of wavelengths, representing the epoch of the universe in which this absorption was occurring.

And that dip is exactly what Bowman and team saw.

Their edges experiment is part of the Murchison Radio-Astronomy Observatory in Western Australia.

This is one of the most radio quiet locations on the planet, far from any human-made interference.

That's because is remote, not because Australians don't have radio yet, despite the rumors.

So the research team added together the CMB light from the entire visible sky and recorded this spectrum.

The dip shows the drop in CMB light due to 21 centimeter absorption.

The wavelength range of the dip corresponds to the epoch between 180 to 270 million years after the Big Bang.

That period represents the time between the birth of the very first stars to the onset of very active black hole growth.

Measuring this range in itself, is a stunning discovery that will really help us understand the early universe.

It was also, expected.

The absorption dip was predicted by our cosmological models, and it was right where we thought it would be.

But there is one big discrepancy between model and observation.

The dip is about twice as deep as we expected.

Absorption is happening when we thought it would, but much more of the CMB is being absorbed than we expected.

This suggests that the hydrogen doing the absorbing is a lot colder than we thought to be.

Colder gas is better at absorbing 21 centimeter photons.

But here's the thing.

Our cosmological models can't explain how this early hydrogen gas could possibly be this cold.

We know exactly it's temperature at the moment of the creation of the CMB so there's a limit to how much it could have cooled since then.

This is where the new physics comes in.

In order to cool something down, you need to expose it to something even colder than itself.

Or expand the universe, but that's already been taken into account.

The only thing colder than this ambient hydrogen at the time, was dark matter.

So maybe, the hydrogen lost some of its heat to dark matter.

Yet, in order for that to happen, hydrogen would actually need to interact with the dark matter, and that's the whole thing about dark matter.

It doesn't interact with regular matter, except through gravity.

But in order to cool the hydrogen, there must be another type of interaction.

This is getting physicists excited.

It's only a hypothesized explanation for the relative coolness of this gas, but it's the one the authors seem to like.

More time and more data will help sort this out.

So now for a complete change of topic.

Let's do the answer to our recent Trebuchet Challenge.

You were a medieval warlord.

Well, maybe early Renaissance-- whatever you like.

We looked at a couple of different scenarios in which you trebucheted your enemies fortress.

And I asked you to use energy methods to figure some stuff out.

First, I asked you the following.

You fire your trebuchet at your enemies wall, twice.

In the first case, the projectile flies upwards on a shallow path, to strike the top of the wall.

And in the second, the projectile flies high in the air to fall again, striking the same location.

In both cases, the trebuchet counterweight started at the same height and also, reached the same height at the end of its swing.

My question was, which shot was the most damaging, assuming damage only depends on the kinetic energy of the projectile at impact?

To answer this, we need to know how much of the counterweights starting potential energy ends up in the projectile.

We know that the counterweights height in both shots was the same at the start and at the end of the swing.

At both of these points, the weight is momentarily still.

It has no kinetic energy, and so, all of its energy is in potential energy.

So the energy it lost to the projectile, is just the difference between these potential energies.

That's the same for both shots so both gave the projectile the same total energy.

We don't know anything about the kinetical potential energies at the moment of release, but we do know that the final potential energies of the projectile in both shots were the same because they hit the wall at the same heights.

That means the projectiles kinetic energies at the point of impact must also be the same.

And as long as they had the same mass, their speeds would be the same, too.

For the extra credit question, I asked, for the speed of the impact of the projectile, assuming the parameters you see on screen now.

Some of these were red herring parameters.

The only things you needed to know where the starting and final heights of the counterweight and projectile and the mass of the counterweight and projectile.

This is the power of using energy in calculations.

So many irrelevant complications melt away.

Like I just explained, we can equate the energy lost by the counterweight with the energy gained by the projectile.

Then, subtract the potential energy of the projectile at its point of impact, and we have its kinetic energy.

Then, half mv squared gives us its velocity, around 80 meters per second.

The kinetic energy of a 90 kilogram stone at that speed is about that of a third of a stick of dynamite.

Hey, it's not bad for a medieval rock slinger.

So we chose six correct answers to receive "Space Time" t-shirts.

If you see a name below, that means you.

Email us at pbsspacetime@gmail.com, with your name, address, US t-shirt size-- small, medium, large, or extra large-- and let us know which tee you'd like.

That includes new heat death of the universe is coming shirt.

If you didn't win this time, there's a link in the description so you can grab your own t-shirt any way.

That way when you do win next time, you can get our upcoming t-shirt, which will be even cooler, if that's possible.

Nice way to show your appreciation for PBS "Space Time."