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Dec 5th, 2016

Which of these is NOT in the Local Group?

  • A. The Magellanic Clouds
  • B. The Andromeda Galaxy (M31)
  • C. The Triangulum Galaxy (M33)
  • D. The Whirlpool Galaxy
  • E. The Canis Minor Dwarf Galaxy

The Whirlpool Galaxy is not within the Local Group! The Whirlpool is in the M51 Group.

What is Dark Matter?

Most matter in the Universe is not ordinary matter but dark matter. We infer the existence of this dark matter from its gravitational pull - not from visual (or direct) ways.

  • It's name indicates that it doesn't emit light.

Measuring Galatic Mass from Galatic Rotation

In the last lecture we discussed the galatic rotation and its relationship with galatic mass - using the equation below.

We could measure galatic mass with from galatic rotation.

M(_<R) = (V^2 * R) / G

Where:

  • M is the mass notation.
    • _<R specifies the mass to be "mass within a certain radius"
  • V is the velocity for a certain astronomical object (say, the Sun; however usually calculated to be 220 km/s).
  • R is the distance (radius) from the center of the galaxy.
  • G is the standard G.

And in the case of a Flat Galatic Rotation, this is a strong suggestion to the existence of dark matter. Similar phenomenom was also discovered in other spiral galaxies. Incorporating gas, stars, and dark matter, a rotation curve shows what we discovered.

But still, dark matter is still holds a hypothesized existence.

Conclusion: Dark matter "halo" contains ~10 times the mass of ordinary matter.

Dark Matters in Clusters

The mass of a cluster can be estimated from the relative velocities of galaxies in the cluster. By using this approach, we find masses ~100 times larger than predicted by combined starlight.

The intra-cluster medium explains some of the missing mass, but we still must invoke dark matter, with a mass of ~10 times the ordinary matter - agreeing with the case of the Milky Way Galaxy.

Dark matters varies amoothly throughout cluster, with concentrations around the individual galaxies.

Another Way to Detect Mass

Recall, Quasars are "stars" - that are actually bright cores of galaxies (Seyfert). Sometimes we were able to see "two" quasars, due to Gravitational Lensing. By measuring the lensing, we could determine the cluster mass. Providing an independent confirmation of large amounts of dark matter.

See Einstein Cross.

The Nature of Dark Matter

See Bullet Cluster.

The case of Bullet Cluster shows that dark matter went "through" in a collision. Dark matter does not fill up space like gas, but in another way that we had not yet been able to identify.

The MACHO Hypothesis

Or MAssive Compact Halo Objet - planets, old white dwarfs, and neutron stars, black holes.

Searches for MACHOs are based on gravitational lensing of stars in nearby galaxies (say, Magellanic Clouds). Passage of a MACHO in front of background star causes a brief brightening. Evidence so far indicates that MACHOs can only account for a small fraction of dark matter "halo".

The WIMP Hypothesis

Or Weakly Interacting Massive Particles.

Massive cousins to neutrinos - and similarly, they don't interact with ordinary matters often. Some experiments (e.g. aboard the International Space Station) hint at WIMPs.

Leading hyptohesis among others, but evidence is still inconclusive.

Cosmology

Big questions in our course:

  • Is the Universe infinite, or does it have an edge?
  • Has it always existed, or did it begin at a point in the pass?

And...

  • The afterglow from the Big Bang can be detected today.
  • By studying this afterglow, we can learn more about the beginning of the Universe.

Hubble's Law: Are we at the center?

No, this is an expansion with no center.

The Cosmological Principle

A universe obeying Hubble's Law has no preferred center of expansion:

The Cosmological Principle: There are no special position in the Universe.

It does not make sense to think of the universe's expansion as resulting from an explosion at some point - instead, general relativity tells us that the expansion of the Universe acts more like a rubberband.

Redshift in an Expanding Universe

The Hubble's Law Redshift of Galaxies is not due to their motion, but due to the expansion of space. As space expands, the distance between successive crests of electromagnetic waves increases - effectively lengthening the waves.