What forms the “texture” of the universe?

Figure 25.29 from book

Why are there filaments and voids in the distribution of galaxies?

Back to galaxies: the rotation curves of spiral galaxies show that most of the mass is in an unknown form of matter

The further we go out in the universe, the more compelling the evidence for Dark Matter

Clusters of galaxies: galaxies + superhot gas + ?

Galaxies are moving around, subject to a gravitational force far greater than the galaxies can produce

Dark Matter manifests itself by trapping hot gas and causing gravitational lensing

Nature even “separates out” Dark Matter when clusters of galaxies collide

Dark Matter and galaxies

• Dark matter dominates the stars in galaxies

• Galaxies move according to a gravitational force determined by the dark matter

• We believe the filaments of galaxies occur because galaxies “trace out” the distribution of dark matter

What is the Dark Matter?

• Hydrogen gas in any form? No, we would have seen it.

• Dead stars of some sort? Probably not; we don’t see the tons of red dwarf stars formed at the same time.

• Let’s get desperate; huge snowballs or solid objects in space? We don’t see massive “hyperbolic” comets in the solar system.

Favorite pick at the present: an otherwise unknown form of

elementary particle

• A particle emergent from a speculative theory of physics called supersymmetry; an attempt to unify all of the fundamental forces.

• Supersymmetry predicts that for every known elementary particle, there is a corresponding, presently undetected partner.

• The lightest supersymmetric particle is speculated to have some of the properties of an ideal Dark Matter particle.

• Until we actually detect a supersymmetric particle in the laboratory, this all remains learned speculation.

An alternative possibility: we are using the wrong law of gravity on the scale of

galaxies

The galaxies we see are all the mass that is there, but the gravitational force differs from that in introductory physics

Stay tuned: there will be more on this in comingyears

Cosmology I

• Definition of Cosmology: The scientific study of the universe as a whole; how long ago it came into being, the nature of that beginning, the future destiny of the universe, and the physical laws that govern it.

• Timeliness: One of the main fields of physics and astronomy nowadays. Status now is drastically different than 10-15 years ago.

How we can talk about cosmology….

Large look-back times show the universe when it was different

The scientific basis of modern cosmology

Hubble’s Law

V = H0d

The universe is expanding

A Model Universe

With this “toy” model, you can show:

• Hubble’s Law, v=H0d

• There is no center to the expansion, if you see Hubble’s Law, there is still nothing special about your location

• The physical significance of Hubble’s Constant: the time since the expansion began, thus the age of the universe

What time does the Hubble Constant represent?

If the universe is expanding now, what will it do in the future?

If cosmology is to be a branch of physical science, there must be an underlying

mathematical structure with quantitatively testable predictions

• There are two things going on in the universe• Galaxies are flying apart due to the universal expansion• The force of gravity is acting to pull them back again• The story of the universe is competition between

universal expansion and gravity

How to describe gravity: General Relativity

• Dynamics takes place in a four dimensional spacetime

• Mass induces warping or curvature of spacetime

• Spacetime curvature may also exist in the absence of mass (cosmological constant)

A sermon on General Relativity

In some contexts, spacetime curvature becomes pronounced, and space and time become inextricably

linked or “coupled”. One example is black holes. Another is the universe as a whole. When we look on

cosmological scales c/H0, you cannot view the universe as three dimensional, with time as a parameter. The universe exists as a 4D entity with strong curvature

effects. You can’t even qualitatively understand what is going on if you stick to a 3D view

An analogy

The surface of the Earth is a 2D surface embedded in a 3D space. Locally, the surface of the Earth

looks purely 2D. On scales of the radius of the Earth, the curvature

is unavoidable.