final exam!!! Today we will discuss Cosmology and the

Big Bang Wednesday we will finish the Big Bang,

have a short discussion of Life in the Universe, and review for the final exam

Remember:

“the Great Wall”

Earth

Virgo Cluster

Expanding UniverseToday’s Lecture: purpose & goals

Looking at an ‘empty patch of sky

AST 1002Planets, Stars and Galaxies

1) Hubble’s Law2) Expanding Universe3) Cosmology Assumptions4) The Origin of the

Universe “the Big Bang”5) Fate of the Universe

Galactic Clusters – II SuperClusters

On even greater scales the galaxy clusters themselves form clusters called superclusterssuperclusters that can be tens to hundreds of millions of light years across.

These superclusters resemble “…the froth on soap bubbles…”

in space. Between them lie great voidsgreat voids

containing relatively few galaxies.

Local Group

Virgo Cluster

Fornax Cluster

Ursa Major group

Canes group

Leo II group

Leo I

Virgo III group

10 million ly

Sculptor

Galaxy Collisions Occasionally galaxies collide

Happens most often in centers of rich galactic clusters

Stars that make up the galaxies don’t actually slam into each other

Passage of one galaxy through/near another causes major “stirring”

due to gravitational interaction Causes new activity such as

huge burst of star formation,

creation of AGNs,…

Think of walking around before or after a football

game

very little gas and dust

significant gas and dust – in arms

some gas and dust

Review

no new star formation!!

significant new star formation!! (in the spiral arms, spherical component older!!)

some new star formation (randomly throughout galaxy)

small, irregular, no rotation

fast rotation

slow rotation

Pop II

Pop II

Pop I

Measuring Distances Stereoscopic viewing (Parallax)

only “small” distances nearest few thousand stars in our galaxy

“Standard candles” objects which have ‘known’ luminosities Cepheid variables

variation tells luminosity – good to 65-100 million LY brightest stars in galaxy, size of HII regions, overall

brightness of galaxy – good in steps out to 1-2 billion LY Type Ia supernova

all have same luminosity good to 8-10 billion LY (a few measured

as far away as 12-13 billion LY)

Relies on Comparison to examples of the same type objects in nearby galaxies

History reminder – “Island Universes”

1924 – Edwin Hubble Measured the distances to

several galaxies using the Luminosity-Period relationship of Cepheid variables, discovered by Henrietta Leavitt (1912). (Cepheid variables)

This was conclusive proof that the

galaxy lay well beyond the Milky Way. 2 million l.y. away!

The universe was much larger than previously thought.

Andromeda galaxy

size of the Moon

Calculate the distance to the Andromeda Galaxy using Hubble’s method.

m - Mv = -5 + 5log10(d(parsecs))

Luminosity-Period Relation of Cepheid Variables

Andromeda galaxy

Distance Modulus Formula

Galactic Redshifts Edwin Hubble (1889-1953) and colleagues

measured the spectra (light) of many galaxies found nearly all galaxies are red-shifted

Redshift (Z)

From Ch. 2, (Doppler effect) Z = v/c

(for speeds approaching c, we’ll need a relativistic version

of this equation.)

rest

rest

-

= observed Z

Andromeda galaxy

Z = [(1+ v/c)/(1+ v/c)]1/2 - 1

Hubble’s Law Hubble found the amount of redshift depended

upon the distance the farther away, the greater the redshift

v = H0 x d

What is H0?Hubble’s data

distance to galaxy

Re

ce

ss

ion

al v

elo

cit

y

Bigger Structure Structure bigger than

galaxies Galaxy groups

2-30 galaxies Local Group – contains the

Milky Way Galaxy clusters

100s of galaxies – Virgo Cluster (rich cluster)

Superclusters groups and clusters combined

The Universe is filled with large scale structure

“walls” and “filaments” separated by great voids – (like froth on soap bubbles)

EarthVirgo Cluster

First detailed study done by Margaret Geller & John Hucra, Harvard-Smithsonian Center for Astrophysics

“th

e G

reat

Wall”

Hubble’s Law Holds for essentially all galaxies with measured

redshift and distance THEREFORE, measuring the redshift tells us the

distance redshifts up to 95% of the speed of light have been

measured!! ultraviolet wavelengths can be shifted into the visible, or

infrared

H0 is Hubble’s constant = 70 km/(s Mpc) Mpc = megaparsec

Consequences If everything is moving away from us and things

farther are moving faster Then the Universe is expanding!

This doesn’t mean what you are probably thinking . . . .

Expanding Universe Space itself is expanding, not matter

flying apart within space. Examples:

dots rubber band raisin bread ants on a balloon

It does not mean we are at the center of the Universe

every part of the Universe sees everything moving away from it

Cosmological Redshift

We now know 3 kinds of redshift

Doppler shift due to motion

Gravitational shift due to distortion of space-time by

mass Cosmological shift

due to stretching of space not due to relative motion

as space stretches, the wavelength stretches and becomes longer

rest

rest

-

= observed Z

Relativistic (near speed of light): Z = [(1+ v/c)/(1+ v/c)]1/2 - 1

Non-relativistic (<0.05c): Z = v/c

Looking Back in Time Remember it takes time for light

to reach us travels at 300,000 km/s So we see things “as they were” some

time ago The farther away, the further

back in time we are looking 1 billion LY means looking 1 billion

years back in time So the greater the redshift, the

further back in time redshift of 0.1 is 1.4 billion lightyears

which means we are looking 1.4 billion years into the past

Thinking Back in Time If all galaxies are moving away

from each, then in the past all galaxies were closer to each other.

Going all the way back, it would mean that everything started out at the same point

then began expanding This starting point is called the Big

Bang We can calculate the age of the

Universe using Hubble’s Lawv = H0 x

d d = v/H0

But distance = rate x time(the time here is how long the expansion has been going on The Age of the Universe)

thubble = 1/H0

Big Bang At the beginning of our

Universe, all matter was together in a very compact form

matter was nothing like it is now very “hot”

Then space started expanding things “cooled” eventually it was possible for

normal matter to “condense” out of the hooter form

Big Bang model makes a number of testable predictions….

…continued!

At the Beginning

Originally all the energy (and matter) of the Universe was condensed into an incredibly small

region MUCH smaller than the size of a proton

Energy, matter, space and time were all very different than today

need a new “theory of everything” to understand not yet possible 11-dimensional space??? (models are very weird)

During early expansion, space-time and gravity became separate from energy and mass

particles and antiparticles were being created from energy and annihilating into energy all the time

Glow of the Universe

The early Universe was very hot and dense

glowed with blackbody radiation

but so dense the light kept getting absorbed (opaque)

Eventually the Universe cooled enough to form hydrogen atoms

blackbody radiation could now travel freely That time called “recombination of the

Universe” Light from that time should be all around

us and be detectable. 3K background radiation

This light should be cosmologically redshifted

Mostly into microwave region CMB was first seen in 1960s

Pensias & Wilson (Bell Labs)

– won Nobel prize in physics for this

twenty years after prediction COBE satellite mapped the CMB

measured the spectrum wonderful match between theory and

data Temperature = 2.73K cooled glow from recombination

era. Incredibly uniform across sky.

Cosmic Microwave Background (CMB)

Composition of Light Elements

Helium

Big Bang model predicts the percentage of light elements

Hydrogen (1H), deuterium (heavy hydrogen, 2H), helium (4He), lithium (7Li), beryllium (9Be), boron (10B and 11B),…

elements formed before recombination (created out of light!)

percentages depend upon density and temperature of early Universe, and how fast it cooled.

Observed percentages agree with Big Bang model predictions

Almost all was created as hydrogen (1H) and helium (4He), with only trace amounts of anything else.

must have cooled from something very hot.

Fate of the Universe The Universe is expanding But gravity should be pulling it back in So what should the Universe’s fate be:

Continue expanding forever Have expansion keep getting slower forever and stop at

infinite time Expansion stops and eventually Universe collapses

upon itself These possibilities are called

open universe flat universe closed universe

Enough Matter? The amount of matter in the Universe helps

determine its fate if there is enough mass, gravity wins given H0 = 70 km/(s Mpc), critical mass density is 8x10-27

kg/m3

define MASS as the actual density of mass in the Universe divided by the critical density

MASS < 1 is an open universe MASS = 1 is a flat universe MASS > 1 is a closed universe

Enough Matter? Visible matter (stars in galaxies, & hot gas)

only 2% of critical density; MASS = 0.02 Dark matter in galaxies

(measured by galaxy rotation curves) about 10 times as much; MASS = 0.2

Dark matter between galaxies (measured by watching galaxies fall

inward in galactic clusters and from gravitational lensing)

raises total to 30% of critical density MASS = 0.3

We do not observe enough matter to cause the Universe to be closed

But it’s not the end of the story…

Is the Expansion Slowing Down?

Answer: Strangely enough…the rate of expansion is speeding up!

Use Type 1a supernovae a standard candle use brightness to determine

distance use redshift to determine

speed compare them data lies below prediction

(galaxies are speeding up!!)

Formation of Structure (early in the Universe) Normal matter was spread fairly

evenly due to interactions and radiation

Dark matter was not distributed smoothly

clumps remained Expansion spread things out

but gravity held large clumps of dark matter together

Dark matter attracted normal matter

source of galaxies and structure