Download Why is the darkness of the night sky evidence for the Big Bang?

Homework:
To prepare for TMT discussions
next week, you should read the
following article critically…
http://pulitzercenter.org/reporting/betweenearth-and-sky
Don’t forget to find me at the IfA open house on
Sunday for extra credit
Wormholes
The simplest example
1
R00  g00 R  T00
2
Has solution (the Schwarzschild metric):
2
dr
ds2  1  (2m / r )dt 2 
1  ( 2m / r )
 r 2  d 2  sin 2 d 2 

The r = 2m singularity is more complicated than
previously mentioned…

This is what the vicinity of the space-like singularity
looks like (with 2 dimensions suppressed):
The solution actually has two parts, the black hole part in one universe where matter
flows in and essentially a white hole part in another universe where matter flows out.
The connection is unstable and the gravitational attraction pinches off the wormhole
A wormhole could be kept open by filling its throat, or the region around it, with
an ingredient called exotic matter. This is strange stuff which doesn’t exist in
the classical physics world, but may be allowed by quantum physics.
Exotic matter is repelled, rather than attracted, by gravity and is said to have
negative energy - meaning it has even less than empty space.
Scientists like John Wheeler and particularly Kip Thorne, realized
that Einstein’s laws of gravity do not:
1. determine the topology of the spacetime – that is determined by
the matter distribution.
2. give a direction to time.
What does 1) mean?
Topologically, a coffee cup is the same as a donut!!

Since Einstein’s equations don’t determine the topology of space time,
then maybe we can fold it and do this:
A jet emerging from a wormhole!!! Not!!!!
Scientists like John Wheeler and particularly Kip Thorne, realized
that Einstein’s laws of gravity:
1. Do not determine the topology of the spacetime – that is
determined by the matter distribution.
2. Do not give a direction to time.
What does 2) mean?
We could use
wormholes to travel in
time!
Heres one scheme – A
traveler could move
from event A to B using
stationary Wormholes
and then back in time
using a moving
wormhole.
Critical thinking exercise – Why won’t these schemes work?
Exotic matter
Could use anything – fairy godmothers, aliens, mental
telepathy, inflation, dark energy…
Direction of time
The laws of Thermodynamics give time a direction – gravity
must obey thermodynamics
What do we mean by dark matter
and dark energy?
Dark Stuff in the Universe
Dark matter
Not dark matter
Unseen Influences
Dark Matter: An undetected form of mass that
emits little or no light but whose existence we
infer from its gravitational influence
Dark Energy: An unknown form of energy that
seems to be the source of a repulsive force causing
the expansion of the universe to accelerate
For groups of stars, galaxies, clusters of galaxies,
we measure the velocities of the member objects,
then assume there has to be enough mass there for gravity to stop them from
flying apart. When we measure these velocities, it turns out that more mass is
needed!
So we assume there is dark stuff.
The first to observe this behavior was Fritz Zwicky in 1933. He was
looking at galaxies in clusters.
No one believed him.
He was quite a character!
What is the evidence for dark
matter in clusters of galaxies?
We can
measure the
velocities of
galaxies in a
cluster from
their Doppler
shifts
The mass we
find from
galaxy
motions in a
cluster is
about
50 times
larger than
the mass in
stars!
Clusters contain
large amounts of Xray emitting hot gas
Temperature of hot
gas (particle
motions) tells us
cluster mass:
85% dark matter
13% hot gas
2% stars
Gravitational lensing, the bending of light rays by
gravity, can also tell us a cluster’s mass
All three methods of measuring cluster mass indicate
similar amounts of dark matter
Vera Rubin detected this lack of luminous stuff in
individual galaxies in the early seventies.
No one believed her!
She is still a character!
Mass within Sun’s
orbit:
1.0 x 1011 MSun
Total mass:
~1012 MSun
Spiral galaxies all tend to have flat rotation curves
indicating large amounts of dark matter
The visible
portion of a
galaxy lies
deep in the
heart of a
large halo of
dark matter
What is the Dark Matter?
Some Possibilities:
Ordinary Matter:
Dust, Gas, Planets, Jupiters, Brown Dwarfs, Faint Stars, White Dwarfs,
bowling balls, black holes, MACHOs …
Current theories/observations say there just isn’t enough mass
possible.
Plus the Hot Big Bang idea doesn’t produce enough “baryonic matter”
(protons, neutrons, electrons etc.) to do it.
Not-so-ordinary Matter:
Neutrinos are an example of a particle that interacts with matter
through the weak nuclear force. There are so many neutrinos in the
universe, however, that even a small mass would be important for
dark matter; a mass of 92 eV, one five-thousandth the mass of the
electron, would close the universe!
WIMPS
Most exotic matter candidates fall into the category of
WIMPS, or Weakly Interacting Massive Particles. These are
heavy particles that only interact weakly with other matter
(or else they would have been discovered by now).
There are many possible – e.g., neutralinos, axions, etc.
Change Gravity:
Another possibility – Perhaps gravity on large scales, such as the size of
galaxies, doesn't work the same way as gravity does on the small scales
we can measure.
MOND – MOdified Newtonian Dynamics
Milgrom
Bekenstein
Sanders
Change Gravity:
MOND – MOdified Newtonian Dynamics
F  ma
a
F  m  a
 a0 
 ( x)  1 if x 1
 ( x)  x if x 1
Stars etc. 10%
Dark Matter 90%