Visible universe is an infinitesimal fraction of the material from the big bang

What we know about the universe today

Edwin Hubble at Mt. Wilson

Hubble guiding the Hooker100 inch telescope in 1923.

The Hooker 100 inch telescope atop Mt. Wilsonnear Pasadena, CA. It was the largest telescope in the world from 1917-1947.

Photos courtesy Mt. Wilson: http://www.mtwilson.edu/History

Hubble’s observations at the 100 inch during the1920’s led him to the conclusion that the universe is expanding, and that an object’s recession velocityis proportional to its distance from the observer.

Photo courtesy Colleen Gino of Mt. Wilson

The View from Mt. Wilson

The Hubble Space Telescope

Thermal and Non-thermal Emission

Light– wavelength , frequency , speed c

Dispersion: Use (prism or grating) to determine spectrum Two kinds of sources

– thermal emission: incandescent heated filament: blackbody radiation…the Planck Law

– non-thermal emission: eg: excited gas emits light at specific colors or wavelength. eg laser, radio, line spectra

pattern of emission lines provides unique fingerprint for each element

Identify spectral lines in distant galaxies– universality of spectral lines: laws of physics invariant in space and time.

c

E hf

E hc

Using the Doppler Effect to Measure Velocity

Doppler Shift- applies to sound and light

» If source is moving with respect to observer, the observer experiences a shift in wavelength

» Velocities away from observer shift light to longer (redshift)

» Velocities toward observer shift light to shorter (blueshift)

» The higher the velocity the larger the shift- provides velocity measure

Blueshift Redshift

f f (1 v /c)

1 v 2 /c 2

f f (1 v /c)

1 v 2 /c 2

z

1

z 1now

then

zNo recession: z=0v=0.5c, z=0.86Most distant galaxy:z=6Cosmic Background: z=1000z=/=a/a where a is characteristic scale size of universe

Galaxy Spectroscopy

Spectra of a nearby star and a distant galaxy

» Star is nearby, approximately at rest

» Galaxy is distant, traveling away from us at 12,000 km/s

Wavelength In

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sity

Sp

ectrum

courtesy B

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Calcium

Magnesium

Sodium

Galaxy Spectrum

Stellar Spectrum

Spectra of nearby and distant galaxies

» Nearby galaxy travels at 261 km/s

» Distant galaxy travels at 6,400 km/s

Astronomical Distance Measurement

»Apparent size of a “standard ruler”– Standard ruler is an object whose intrinsic size is known

– Apparent (angular) size provides distance d given intrinsic size r

»Apparent brightness of a “standard candle”– Standard candle is source whose intrinsic brightness is known

– Apparent brightness b provides distance d given intrinsic brightness B

r

d

d r

tan r

b B4d 2 d B

4b

Standard Rulers in Everyday Life

The STOP sign is an everyday “standardruler”. If we know STOP signs are all the same size, the apparent size of a STOP sign provides us with distance information.

Some Known Standard Candles and Rulers

»Standard rulers– Elliptical galaxies

– Galaxy clusters

»Standard candles– Certain types of stars (Cepheid variables)

– Spiral galaxies

– Certain types of supernovae (Type Ia SNe) Exploding white dwarfs Emit as much light as an entire galaxy, so can be detected at great

distances

Type Ia Supernova 1998bu in M96

Observations from the CfA Supernova Group: Kirshner, Garnavich, Challis and Jha

SNe look like bright stars superimposed on galaxies. They brighten toward maximumand then fade away over time as the hot material expands and cools.

Type Ia SupernovaObservations from the CfA Supernova Group: Kirshner, Garnavich, Challis and Jha

Repeated observations yield a light curve- measured brightness versus time- whichastronomers use to determine the peakapparent brightness and distance of theSN and parent galaxy. This is SN 1998aq.

A spectrum of the light emitted bySN 1998bu. The features in this spec-trum identify SN 1998bu as Type Ia.

Testing Expansion with Type Ia Supernovae

Find SNe in distant galaxies (rare objects)

Take spectrum to confirm they are Type Ia SNe

For each SN Ia

» Measure the recession velocity of the parent galaxy vr

» Measure the maximum apparent brightness and compare that to the intrinsic brightness to calculate the distance d

» Place that point on Hubble diagram

Hubble Diagram

Distance

Vel

ocit

yA single supernova

vr

d

Type Ia Supernovae Measurements

Distance measurements to 19 SNe

Riess, Press & Kirshner ApJ 1996

Blue points: 19 SNeRed line: Hubble Law with Ho=19.6 km/s/Mly =64km/s/Mpc

Type Ia supernovae and every other distance indicator usedprovides results consistent withthe Hubble Law: other galaxiesare receding from us, and theirrecession velocities are pro-portional to their distances, inother words, the farther awaythe galaxy, the faster it travelsaway from us.

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Interpreting the ExpansionGalaxies are receding from us, and their recession velocities

are proportional to their distances from us

Two interpretations»Bad neighbour hypothesis

– We are at the center of the universe, and the rest of the universe is trying its best to get away from us.

»Homogeneous expansion hypothesis– The whole universe is expanding, and observers on any other

planet in any other galaxy would note the same proportionality between recession velocity and distance- the Hubble Law.

BNH violates the Cosmological Principle.Only linear homogeneous expansion is universally

internally consistent

SummaryObservations: galaxies tend to travel away from us, and

their recession velocities vr are proportional to their distances d- the Hubble Law:

Our universe is expanding homogeneously and is not static.»The universe was denser in the past»The universe had a beginning….?

Expansion is generic to the Big Bang model.

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What we know and what we don’t

Matter density is dominated by cold dark matter that we know nothing about.

Perturbations which give rise to structure formation arise in the inflation era due to ultra-high energy processes about which we know nothing!

The universe is dominated by a property of space called dark energy, cosmological constant or quintessence (also called the cosmo-illogical constant!). We know nothing about it!

Baryon asymmetry arises in the GUT or Electroweak era due to CP violation but the details are unknown