Transcript The cosmic connection

The cosmic connection
There is a very close
connection between
particle physics and
astrophysics.
I’m going to show two
examples:
Type II supernovas
Dark matter
SN1994
Particle decay
Heavier particles decay into lighter ones, which is why the
world is made of up and down quarks and not top quarks.
As an example, free neutrons decay into protons, electrons
and electron anti-neutrinos:
_
np e e
This example is called beta decay.
Question: why don’t neutrons within an atomic nucleus
decay?
Particle decay
In such decays, several quantities are conserved:
Energy, linear momentum, angular momentum
Electric charge
The total number of leptons (anti-leptons count as -1)
The total number of baryons (anti-baryons count as -1)
_
np e e
We have one baryon to start, one baryon to end.
We have zero leptons to start and zero to end (the antineutrino
counts as -1, the electron as +1)
The “bar” means “antiparticle”
Particle Decay
All decays can go in reverse, too (inverse beta decay)
epne or enpe
General rule: When you move a particle from one side of the
decay equation to the other, it changes into it’s antiparticle
Verify that lepton number and baryon number are conserved in this decay.
Beta decay and supernovas
Why is this important in astrophysics?
Beta decay is crucial in the generation in stars of all the chemical
elements.
And inverse beta decay plays a central role in Type II supernovas
A Type II supernova occurs due
to gravitational collapse
Supernova 1064 remnant
Type II supernovas
In type II supernovas (stars much more
massive than our sun), when all the
nuclear fuel is used up, the gravitational
pressure is so great that the atoms
collapse!
The electrons and protons undergo
inverse beta decay:
epne
Every proton in the star becomes a
neutron, forming a neutron star.
The neutrinos escape(1057 of them!) and
carry away a large part of the supernova
energy.
Galaxy M31 with a SN on
the outter edge
Type II Supernovas
The neutron star resulting from a
Type II supernova is about 10 km
in radius and rotates with periods
of seconds or less.
In 1987 neutrinos from a nearby
supernova were detected in two
detectors on earth!
Here’s a problem for your students: If a star the size of our sun with a rotation
period of 10 hours collapses to a radius of 10km, how fast would it be rotating to
conserve angular momentum?
Dark Matter
As early as 1933 Zwicky showed
that gravitational effects in galaxy
clusters could not be explained by
the visible matter.
In the 1980’s, rotational curves of
galaxies gave more evidence for
the existance of unseen or dark
matter.
Rotational velocity vs distance from the
galactic center
Gravitational Lensing
Einstein predicted in 1936
that light from distant objects
could be bent by massive
objects between us and the
distant object.
Multiple, distorted images are
evidence for gravitational
lensing.
Dark matter
Quantitative calculations of gravitational lensing give strong
evidence that galaxies have much more matter than we see.
Galactic Haloes
The rotational curves of galaxies and the observed
gravitational lensing can be explained if galaxies are
surrounded by a huge halo of dark matter.
The dark matter makes up 80% of the matter in galaxies!
What is the dark matter?
We don’t know, but the answer almost certainly
lies in particle physics.
In the 1980’s particle theorists developed what is
still a leading candidate for a theory beyond the
Standard Model. Supersymmetry predicts that
every particle we know has a supersymmetric
partner which is much more massive and
interacts only weakly with regular matter.
Supersymmetry solves deep problems in the
mathematics of the Standard Model. It was
developed for this reason, and not because it
solves the dark matter problem.
Is Supersymmetry real?
“Supersymmetry is an offer nature can’t refuse”
Dmitri Nanopolis, theorist
“There ain’t no supersymmetry”, Leo Bellantoni,
experimentalist
“Experiment is the sole judge of scientific truth”,
Richard Feynman
We are pushing hard to find supersymmetry. But as
yet there is no direct experimental evidence for it.
Dark Energy
It gets weirder…around 2000 it
was found that the expansion of
the universe is speeding up.
The stuff in the universe is
dominated by dark energy, which
acts like anti-gravity.
Dark energy is pushing the
universe apart faster and faster.
It’s nature is unknown.
The cosmic connection
The science of the very big and the
science of the very small are in a
close, synergistic relationship.
Deployment of the Hubble Space Telescope
from the shuttle
Aerial view of Fermilab