The Expanding Universe No… these will not be on the HUB. If you have special needs, see me individually.
Background • Einstein’s theory of gravity (general relativity) predicted an expanding universe in 1915. • Hubble studied the motion of galaxies in 1929 and found that they are moving in a direction away from each other.
The Theory • Some people believe that the Big Bang is “just a theory” with no evidence to support it. • In science, the word “theory” means an idea that is well-established and supported by scientific evidence… like the Theory of Plate Tectonics. • Hubble’s observations of galaxies moving apart from each other are one of several pieces of evidence that supports the theory.
Redshift Leading to Recessional Velocity • Hubble observed a noticeable displacement of known spectral lines towards the red-end of the spectrum when analyzing spectra from distance galaxies.
Evidence for an expanding universe The spectrum of hydrogen gas is the unique fingerprint of that element Hydrogen lamp
Evidence for an expanding universe When we see a repeat of the pattern we saw in the lab, we know hydrogen is present. The green lines indicate oxygen. Orion Nebula: star forming region in the Milky Way
Evidence for an expanding universe We see the same repeating pattern of lines in a galaxy, but displaced to the red Galaxy UGC 12915: A distant galaxy
Evidence for an expanding universe The further the galaxy, the more the shift to the red Galaxy UGC 12508: RV = 9100 km/s
Evidence for an expanding universe The greater the red shift, the faster the galaxy is receding Galaxy KUG 1750 RV = 15,400 km/s
Conclusion • The redshift displacement is larger for galaxies that appear more faint from Earth. • The farther a galaxy is, the faster it is receding from Earth. • http://hyperphysics.phy-astr.gsu.edu/hbase/astro/redshf.html
Measuring Distance • Distances to open clusters, a collection of young stars with a common motion, near Earth can be determined using radial velocity & proper motion measurements. • We can also use this information to find the luminosity of these stars. • Similar stars are analyzed in more distant clusters. Assuming they have similar properties, their distance can be determined.
Other “standard candles” like Cepheid variables & RR Lyrae stars and supernovae can be analyzed and compared to more distant objects to determine long distances. • Recall there is a relationship between apparent magnitude, luminosity, and distance…
Side Note… Variable stars • Variables are those stars whose brightness changes with a definite pattern. • The variation is caused because of the star puffing out and contracting back again. • The period of oscillation has a relation to its luminosity (thus there is a period-luminosity relation for these stars)
The Hubble Constant • The Hubble Constant (H0) indicates the rate at which the universe is expanding. • Remember it is the slope of the line on a graph of recessional velocity vs distance • The units for the constant are km/s / Mpc. 1 Mpc = 3.26 million light years
Hubble Time • The most recent calculation of the constant done in 2011 is 73.8 km/s/Mpc. • The constant has units of inverse time or 1/s. So… after a lot of unit conversions… • We get that the universe is 13.7 billion years old.
Background RadiationAKA… leftover heat • Penzias & Wilson encountered “noise” from an antenna used for radio astronomy in 1965. • According to the theory, the universe would have been extremely hot and dense in the beginning. • It would cool as it expanded.
The temperature can be calculated to be 2.725 K • Recall that the peak wavelength or wavelength of maximum intensity corresponds to temperature (and color for stars).
Testing the Big Bang model Prediction: A hot, dense expanding universe, should be predominantly hydrogen, helium. Observation: Universe is ~75% hydrogen, ~25% helium by mass Cecilia Payne 1925 The Sun: 74.5% H, 24% He by mass
Composition • Hydrogen fuses to helium in the early “days” of the universe. • Expansion & cooling causes the nuclear furnace to shut down. • All other elements are created in the life cycle of stars. • The theory suggests a 12:1 ratio H to He by number of atoms or 75% to 25% by mass.
Testing the Big Bang model Prediction: An expanding universe is evolving over time. If we look at the early universe, it should appear different. Observation: Distant galaxies less evolved, physically and chemically.
Evolution of Galaxies over Time • Looking into space is looking back in time. • Distance galaxies appear as they did when the light left them billions of years ago. • Early evidence came from radio surveys but the HST has provided the deepest views of the universe.