expanding universe Flashcards
The Milky Way and
Andromeda are part of
a small cluster of
galaxies called
the
‘Local Group.
The nearest large
cluster to the Local
Group is
the Virgo
Cluster.
21 out of 25 spirals showed a
systematic
redshift:
* Systematic motion away from us.
* Some velocities are large: >2000 km/sec
1920’s: Two theorists used Einstein’s
Theory of General Relativity
to find that a
stable Universe must expand.
* Alexander Friedmann (1922)
* George Lemaître (1927)
Lemaître used Slipher’s results to support
his prediction
1929: Edwin Hubble measured distances to
25 galaxies:
Used Cepheids in Andromeda & Local Group
* Used brightest stars in the other galaxies
* Compared distances and recession velocities
Using Cepheid variable stars,
Hubble (1929) found that
more distant galaxies have
greater redshifts, and
therefore higher velocities.
The greater the distance of a galaxy from Earth
the greater the observed redshift
The 1929 work was very suggestive of an
effect, but the uncertainties were large.
Set about collecting more data:
By 1931
Added 8 more galaxies with good distances
* The most distant galaxy had a recession
velocity of nearly 20,000 km/sec!
* Showed a stronger, tighter trend with distance
The Universe is expanding in a systematic way
The more distant a galaxy is, the faster it appears
to be moving away from us.
Hubble Constant: Rate of expansion today.
According to the Hubble Law, galaxies twice as far away
move ____ us _____ faster
away from; 2 times
If the Hubble constant is 70 km/sec/Mpc, then a galaxy
with a radial velocity of 7000 km/sec is at a distance of
100 Mpc.
General Expansion of Spacetime
Observers in different galaxies see the same expansion around them.
* No center - all observers appear to be at the center.
What is the recession velocity
NOT motions through space…
* Expansion of spacetime: galaxies carried along
1 AU =
Mean Earth-Sun Distance
Method: Geometric Triangulation
Radar bounced off inner planets.
* Orbits of planets give the geometry.
* Works out to ~50 AU
Step 1:
The Astronomical Unit
Step 2: Trigonometric Parallaxes
Calibrated by the AU (size of Earth’s orbit).
Method: Stellar Parallaxes
* Use Earth as the baseline.
* Ground-based: out to ~100 pc
* Hipparcos: out to ~1000 pc
* Gaia: goal is ~10,000 pc
Gives measurements of:
* Luminosities of nearby stars
* Distances to nearby star clusters
Step 3: Spectroscopic Distances
Calibrated by Trigonometric Parallaxes.
Method: Spectroscopic Distances
* Relate spectral type to luminosity in calibrated H-R
Diagrams.
* Works OK for individual stars.
* Works best for clusters of stars
Gives measurements of:
* distances to clusters out to ~50-60 kpc
Step 4: Cepheids
Calibrated by cluster H-R diagrams.
Method: Period-Luminosity Relation
* Cepheids: supergiants in young clusters.
* Calibrate the Cepheid Period-Luminosity
relation in the LMC
Cepheids give distances to:
* Nearby spiral galaxies out to 30-40 Mpc.
Only works for spirals (need Pop I stars).
What is a ‘standard candle?’
an object for which we are likely to know the true
luminosity
Step 5: Galaxy Standard Candles
Look for bright standard candles found in
both Spiral and Elliptical galaxies
* White-Dwarf Supernova (Type Ia) explosions
* Planetary Nebula luminosity distribution
* Globular Cluster luminosity distribution
Calibrated by:
* Cepheid Period-Luminosity distances
* Nearby similar objects (from other steps)
Step 5: the bottom line
Variety of techniques get used:
* Mix and match to seek consistent results.
* All rely on previous steps, especially Step 4.
* Argue endlessly about the details.
Bottom Line:
* Work out to 50-3000 Mpc, depending on method
* Gives distances to Virgo Cluster galaxies.
* Gives a local measurement of H0
Step 6: Galaxy Luminosities
Calibrated by the Virgo Cluster distance.
Method:
* Assume distant galaxies are like nearby ones.
* Use correlations between luminosity &
distance-independent properties of galaxies
* Compute luminosity distances using the entire
galaxy.
Seek a refined estimate of H0 at greater
distances.
Step 6: Techniques
Tully-Fisher Relation for Spirals:
* Galaxy Luminosity - Rotation
Speed relation
* Measure rotation speed from
21cm radio emission (distance
independent)
* Limitation: useful only for spiral
galaxies
* Why it works is not well
understood.
