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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.


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.


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.


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


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


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


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,

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."

The Description of Scientists Have Detected the First Stars | Space Time