Transcript Cosmology - Dark Matter, Dark Energy, and Fate

Cosmology
---------------Dark Matter, Dark Energy, and
The Fate of the Universe
© Sierra College Astronomy Department
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Introduction
It is difficult beyond description to
conceive that space can have no end;
but it is more difficult to conceive an end.
It is difficult beyond the power of man to
conceive an eternal duration of what we
call time; but it is more impossible to
conceive a time when there shall be no
time.
Thomas Paine, The Age of Reason (1796)
© Sierra College Astronomy Department
2
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Introduction
Cosmology
– The study of the overall structure and
evolution of the Universe.
– Basic Questions
•
•
•
•
How did the Universe begin?
What is the structure of the Universe?
What is the fate of the Universe?
What observational evidence is there to
support our cosmological ideas and models?
© Sierra College Astronomy Department
3
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Two Important Ingredients
• The Structure and Fate of the Universe
Depend on Two Important Ingredients
– Dark Matter: Discovered in galactic halos, it is
the dominant form of mass in the Universe, but
other than the shape of galactic rotation curves, it
has not been detected, and hence identified, by
any other means. (Possible causes: MACHOs,
black holes, WIMPs, neutrinos)
– Dark Energy: An unidentified form of energy that
is causing the Universe to increase its rate of
expansion with time, contrary to all expectations
before its discovery. (Possible causes: vacuum
energy, cosmological constant, quintessence)
© Sierra College Astronomy Department
4
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Evidence for Dark Matter
• Evidence for Dark Matter in Galaxies
– The Milky Way Galaxy
• The rotation curve of the Galaxy as determined by
21-cm radiation from clouds and optical lines from
stars.
• Sun’s orbit encompasses about 100 billion solar
masses; whereas a distance twice as far
encompasses twice the mass.
• Detailed analysis shows that most of the Galaxy’s
mass is in a spherical halo, tens of thousands of
light-years in radius, and about 10 times the total
mass of the stars in the disk.
• Very little radiation has been detected from this halo.
© Sierra College Astronomy Department
5
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Evidence for Dark Matter
• Evidence for Dark Matter in Galaxies
(continued)
– Other Spiral Galaxies
• Rotation curves determined by measuring 21-cm
radiation from atomic hydrogen clouds since this
radiation can be detected at large distances from
the galactic centers.
• Rotation curve used to determine a galaxy’s total
mass.
• Total luminosity of galaxy used to determine
galaxy’s mass due to stars and this is subtracted
from the total mass to obtain the dark matter mass.
• Typical spiral galaxies are 90% dark matter and
10% stellar matter.
© Sierra College Astronomy Department
6
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Evidence for Dark Matter
• Evidence for Dark Matter in Galaxies
(continued)
– Elliptical Galaxies
• Rotation curves are not possible due to random motion
of stars and 21-cm radiation analysis not possible due
to the lack of gas.
• The random motion of the stars creates a broadened
spectral line and the broader the line the faster the
stellar motion.
• Spectral lines maintain a fairly constant width, and
hence the star velocities remain fairly constant as we
look at greater distances from the galaxy center.
• Consequently, ellipticals show evidence of dark matter.
• Globular clusters orbiting around ellipticals also lend
evidence to the dark matter being present.
© Sierra College Astronomy Department
7
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Evidence for Dark Matter
• Evidence for Dark Matter in Galaxy Clusters
– Orbits of Galaxies in Clusters
• The recessional velocities of a cluster of galaxies are found
and these velocities are used to determine the cluster center.
• The radial velocity of each cluster relative to the cluster
center is then determined, and through an averaging
process, the average orbital velocity of the cluster galaxies is
determined.
• The average orbital velocity of the galaxies then gives
the cluster mass and this mass is compared to the
cluster luminosity.
• Cluster luminosities are found to be far too low for the
amount of mass present.
• The amount of dark matter is found to be up to 50 times more
than the mass in the stars, a multiplication factor significantly
greater than what is found for individual galaxies.
© Sierra College Astronomy Department
8
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Evidence for Dark Matter
• Evidence for Dark Matter in Galaxy Clusters
(continued)
– Hot Gas in Clusters
• Gas within a cluster (intercluster medium) has been found
from X-ray studies to be very hot (10s to 100s of millions of
degrees) and have 7 times as much mass as the stars in the
cluster.
• Since most clusters are is a state of gravitational equilibrium,
the temperature of a cluster’s intercluster medium is
dependent on the cluster’s total mass.
• Specifically, the speed vH (in m/s) at which hydrogen nuclei
move around the center of a cluster given a gas temperature
of T (in Kelvin) is:
m
vH  140
s
x T
• The amount of dark matter is found to be up to 50 times more
than the mass in the stars, the same amount as found with
the orbiting galaxies technique, and is the “glue” that holds
clusters together.
© Sierra College Astronomy Department
9
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Evidence for Dark Matter
• Evidence for Dark Matter in Galaxy Clusters (continued)
– Gravitational Lensing
• All methods to this point are based on Newton’s laws to measure
galaxy and cluster masses.
• General Relativity predicts that mass can “bend” spacetime and this
prediction has been confirmed by numerous observations.
• A consequence of this bending of spacetime is that a concentration
of mass can act like a lens and bend light beams by a process
known as gravitational lensing.
• The amount of bending is directly related to the amount of mass
causing the bending.
• From the analysis of photos showing galaxy clusters (acting as the
mass lenses) and the multiple images of very distant galaxies, the
amount, it is once again found that dark matter exceeds the mass of
the stars by almost 50 times.
© Sierra College Astronomy Department
10
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Is Dark Matter for Real?
• Does Dark Matter Really Exist?
– All evidence for dark matter rests on our
understanding of gravity, either from the Newtonian
view or the Einsteinian view.
– It is possible that our theory of gravity is wrong on
cosmological scales?
– Perhaps, but all attempts to explain dark matter with
alternate theories of gravity, do not succeed in
explaining all the other systems involving mass which
are explained by Newton/Einstein.
– Thus, until some other solid idea comes along to the
contrary, dark matter needs to be taken seriously.
© Sierra College Astronomy Department
11
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
The Stuff of Dark Matter
• Dark Matter Composition
– “Ordinary” Dark Matter
• Most ordinary matter is made of protons and neutrons, which in
turn belong to a group of particles called baryons.
• Objects like you, the planets, brown dwarfs, and faint red mainsequence stars, all baryonic matter, will appear as dark matter
since our technology cannot detect the radiation from them.
– They do not emit enough light even if they were a few light years
away.
– These objects are called MACHOs = MAssive Compact Halo
Objects.
• Lone black holes would also go undetected from observations of
radiation.
• Back-of-the-envelope calculations and probing the Milky Way’s
halo with gravitational lensing techniques (more properly microlensing), shows that MACHOs and black holes cannot account
for all of the dark matter mass.
© Sierra College Astronomy Department
12
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
The Stuff of Dark Matter
• Dark Matter Composition
(continued)
– “Extraordinary” (Non-Baryonic) Dark Matter
• Neutrinos
– Produced in large quantities by stellar fusion.
– No electrical charge and give off no radiation.
– A neutrino has very low mass and is only subject to weak and
gravitation forces.
» Known as a weakly interacting particle.
– Very high speeds do not allow a galaxy to trap them gravitationally.
– Calculations show they can only make up a small part of the dark
matter outside galaxies.
• Other Particles
– What about more massive and weakly interacting particles?
– These types of particles, called Weakly Interacting Massive
Particles (WIMPs) are theoretical.
– Believed to constitute most of the dark matter in the Universe.
– Explains why dark matter in halo and not disk (WIMPs lack a
cooling and collisional mechanisms to bring them into disk).
© Sierra College Astronomy Department
13
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Large-Scale Structure of the Universe
• A Hierarchy of Large-Scale Structure in the Universe
– Galaxies
• Gravitationally bound systems of stars, gas/dust, and dark matter.
• About 100,000 light-years across.
– Groups and clusters
• Gravitationally bound systems of galaxies and dark matter
• Up to 10 million light-years or more across.
• Generally located at the intersection of large-scale galactic chains.
– Superclusters
• Associations of groups/clusters that may be gravitationally bound and in
the early stages of formation.
• Sizes range up to a few 100 million light-years across.
– Chains, sheets, and voids
• May extend up to one billion light-years across.
• On larger scales, Universe finally takes on a more uniform look.
© Sierra College Astronomy Department
14
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Large-Scale Structure Formation
• The Creation of Large-Scale Structure
– Creation of the large-scale structures requires
“centers of gravity” – areas of higher mass density
than surrounding regions to draw matter into “protostructures”.
– Consequently, early Universe could not have been
perfectly uniform in mass density.
– The existence of large amounts of dark matter
suggests that high density regions of dark matter in
the early Universe acted as the catalysts for the
creation and evolution of the large-scale structures.
© Sierra College Astronomy Department
15
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Large-Scale Structure Evolution
• Hierarchical Structure Formation and Evolution
– The large scale structures are believed to form from
a bottom-up approach – galaxies form first, then
clusters, etc.
– Gravitationally bound systems (galaxies,
clusters/groups, and perhaps superclusters) do not
engage in the expansion of the Universe.
• Space is not expanding within bound systems.
• Expansion occurs primarily in the voids and unbound
structures (portions of the chains and sheets and
superclusters)
© Sierra College Astronomy Department
16
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
The Fate of the Universe
• Gravity and the Ultimate Fate of the Universe
– Observations show that the Universe is expanding
– The gravitational attraction of the mass in the
Universe seems to suggest two possible futures
• Gravity is strong enough to slow the Universe to a halt,
initiate its collapse, and then bring the Universe to a “Big
Crunch”.
• Gravity is not strong enough and only slows the
expansion, never ending it. The Universe simply expands
“out of existence” as it becomes a cold dark place.
– So what will it be: Death by fire or Death by ice?
© Sierra College Astronomy Department
17
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
The Fate of the Classical Universe
• A “Classical” Picture of the Universe’s Fate (Dark Energy Not considered)
– The fate of the Universe rests on how “strong” gravity is and this
strength is dependent on the mass density of the Universe.
– For the current observed expansion rate of the Universe, the
theoretical critical density is found to be 10-29 g/cc.
• For actual densities greater than the critical density, the Universe will
eventually collapse.
• For actual densities less than or equal to the critical density, the
Universe will expand forever.
– The observed density of the Universe, determined from just the
luminous matter is found to be 0.5% of the critical density, this is
200 times short of what is needed to halt the expansion.
– Assuming that dark matter may contribute up to 50 times more
mass to the Universe, the actual density of the Universe may be
about 25% of critical.
– The Universe appears destined to expand forever.
© Sierra College Astronomy Department
18
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
The Fate of the “Real” Universe
•
Discovery of Dark Energy in 1990s
– Using white dwarf supernovae to determine the distances to very distant
objects, Universe was found (surprisingly) to be accelerating with time.
– A repulsive force, referred to as dark energy, was hypothesized as the agent
of the acceleration.
•
With dark energy possibility, four fates of the Universe are envisioned:
– Recollapsing Universe
• No dark energy, matter density exceeds critical
• Universe is closed (finite) and ends in Big Crunch
– Critical Universe
• No dark energy, matter density equals critical
• Universe is flat (infinite) and stops expanding at infinite time
– Coasting Universe
• No dark energy, matter density less than critical
• Universe is open (infinite) and never stops although it continuously slows
– Accelerating Universe
• Finite dark energy (that does not evolve in time)
• Geometry of spacetime dependent on mass density
• Universe expansion accelerates with time
© Sierra College Astronomy Department
19
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Details on Fate
• More on the Dark Energy Observations
– To determine which of the four Universes we live in
(recollapsing, critical, coasting, and accelerating), theoretical
curves of all four are first plotted on a graph of “average
distance between galaxies” vs “time before and after now”.
– Supernovae data points are then placed on the graph
• Distance measurements give lookback time.
• Redshift gives how much Universe has expanded since time of
supernova explosion.
– Data appears to match accelerating Universe the best.
– It appears the fate of the Universe is to expand forever.
• The accelerating force of dark energy may eventually rip all
matter apart.
• Mystery of what dark energy is keeps astronomers from being
absolutely sure of the Universe’s fate (will dark energy evolve
away with time and allow gravity to regain control?)
© Sierra College Astronomy Department
20