What is particle astrophysics?

Particle astrophysics is astrophysics studied with particles other than light.

When it comes to traveling the largest cosmic distances at the highest energies, light is not the most reliable messenger of astrophysics.


astrophysical messengers
Astrophysical messengers. Pictures are all borrowed from Fermi, IceCube and LIGO collaborations, and the Internet.


We live in a boring part of the Universe. 

This allows life and the life sciences to thrive here. 

However, everything that is interesting (to me) in astrophysics takes place far, far away. 

For example, most Gamma Ray Bursts take place about 1 Gigaparsec away from us. 

That is over three billion light years away! 

To give you an idea of the scale, our galaxy is about 100,000 light years across.

That is, it takes light a hundred thousand years to get from one end of our galaxy to the other. 

Why do I care about Gamma Ray Bursts?

Well, in short, they are Nature's most powerful accelerators and they outshine an entire galaxy when they occur!

They are super duper explosions. 

What is more, the physics behind these exotic events continue to remain mysterious for over 50 years. 

(Gamma Ray Bursts were discovered during the Cold War by the U.S. military)

Traditional astrophysical messengers are not able to completely probe physics that take place at the farthest distances and at the highest energies. 

This is why we need new messengers or previously unused particles.

Since the beginning of astronomy, we have relied on optical light to study objects in the sky. 

This is the light that our eyes can see. 

In the last few decades, we have started utilizing light of other wavelengths such as X-rays and gamma rays. 

X-rays and gamma rays have shorter wavelengths than optical light and therefore higher frequencies.

Higher frequency light is more energetic than lower frequency light.

High energy light can directly convert to matter and get absorbed by matter.

Light of energy 13.6 eV gets absorbed by Hydrogen atoms, the most abundant element in the Universe.

So, there is an inevitable need for complementary messengers.

Fortunately, in the last century, we have opened up multiple new windows to peer into the Universe. 

About a 100 years ago, cosmic rays were discovered by Victor Hess in a balloon-based experiment. 

These are charged particles hitting the earth all the time.

In the last several years, the IceCube neutrino observatory has discovered the first astrophysical neutrinos up to energies of a few PeV.

1 PeV = 10^15 eV

These neutrinos are from other galaxies where more exciting things are happening.

Moreover, gravitational waves were discovered by the LIGO collaboration in the last few years, confirming, for the first time, the association of short Gamma Ray Bursts with neutron star-neutron star mergers.

Neutron stars are very compact objects (10 km radius!) that form when a star dies.


Neutrinos: my favorite space particles


Neutrinos are so light that for a long time they were thought to be massless.

They are potentially perfect candidates for carrying information about distant particle accelerators all the way to us. 

This is because these elementary particles are neutral and weakly interacting!

Unlike cosmic rays, they can't get bent around by magnetic fields because they are neutrally charged!

So neutrinos remain unattenuated and point straight back to their source. 

Neutrinos are the side product of almost every nuclear reaction and can carry versatile information about particle physics taking place at cosmic distances. 

Our sun makes neutrinos! But, they are low-energy and not very interesting (to me). 

The neutrinos I am interested in are ultra-high-energy neutrinos.

Ultra-high-energy means > 10^18 eV!

Such neutrinos may be associated with powerful phenomena like Gamma Ray Bursts and help us to understand the physics driving these super duper astrophysical phenomena!

So, that's particle astrophysics. 

It is the study of astrophysics that happens so far away that we need the least extroverted particles to carry information about them to us. 

Particles that don't stop to talk to other particles.

Particles like neutrinos.

Post any comments or questions below!

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