Cosmology Dr Katy Lancaster
Overview • The Big Bang • Formation of the Universe • The Cosmic Microwave Background • The COBE experiment • The Expanding Universe • Redshift • Hubble’s Law • The age of the Universe • The Fate of the Universe • The critical density • Big crunch or big chill?
Cosmology:‘The science or theory of the universe as an ordered whole, and of the general laws which govern it. Also, a particular account or system of the universe and its laws.’
The Cosmological principle:‘The Universe, on average, looks the same from any point and in all directions.’ • Can observe the local Universe and draw conclusions about the Universe as a whole
The Big Bang
BOOM! EVERYTHING! IN THE BEGINNING…….
The Big Bang • Not really an ‘explosion’ • Universe expanded rapidly as a whole • Universe is still expanding today as a result of the Big Bang • Matter was created in the form of tiny particles (protons, neutrons, electrons) • Too hot for normal ‘stuff’ to form (eg atoms, molecules)
COSMIC ‘SOUP’ PROTON NEUTRON ELECTRON Charged particles - photons scatter (like ‘fog’)
300,000 years later Much cooled, atoms form, photons escape
Formation of the CMB • The Universe is initially hot, dense and ionised • Photons continually scatter from charged particles until…. • ….temperature decreases and atoms form (neutral particles) • Photons ‘escape’ and stream freely through the Universe. • Observe the same photons today, much cooled, as the Cosmic Microwave Background
Discovery of the CMB • Penzias and Wilson record excess noise when observing the galaxy • Soon identified as the CMB • Isotropic to 1 part in 100,000 - recognised as evidence for the big bang
What is the CMB like? • Can observe the CMB photons today, 13.7Gyr after the Big Bang • Radiation has been highly redshifted by the Hubble Expansion (wavelength now longer) • Much cooled: 2.73 K (compare this with 3000K at recombination) • Conclusive evidence for the Big Bang theory - proves Universe was once in thermal equilibrium • So..... what does it look like?
The Cosmic Background Explorer (COBE) • First satellite dedicated to CMB research • Launched by NASA in 1989 • Measured the spectrum of the CMB • Also measured temperature fluctuations • Scientists won the Nobel Prize in 2007! (15 years after releasing their results)
A perfect blackbody Remnant heat of the creation of the Universe
Observe ‘blank’ sky with a radio telescope (eg COBE) • Rather than darkness, see Uniform, high-energy glow • High sensitivity measurements reveal......
Tiny temperature differences • When the CMB photons ‘escaped’, structures were starting to form • These structures have now become galaxies • The structure formation processes have affected the CMB and we see the imprint as ‘hot’ and ‘cold’ spots • Very difficult to measure!
What does the CMB tell us? • Measure the strength of the temperature differences on different scales, eg:
What does the CMB tell us? • Measure the strength of the temperature differences on different scales, eg:
Parameters • The function on the previous slide is complex and involves many parameters including: • The Hubble constant • The density of the Universe • The curvature of the Universe • The age of the Universe • And more….. • We can constrain some of the big questions in cosmology by observing the CMB • We will look at some of these in more detail later
Back to the expanding Universe…..
Edwin Hubble • Edwin Hubble discovered objects beyond the Milky Way - Andromeda • He realised that these objects are all moving away from us • By the cosmological principle, we see that all objects are moving away from each other
Balloon Analogy
Redshift • The light from a galaxy which is moving away from us will be Doppler Shifted • As the object is moving away from us, the light will be shifted towards the red end of the spectrum • We refer to this as the cosmological redshift for objects moving along with the general expansion of the Universe
Redshift - Blueshift
Redshift Equations • Redshift is defined as the change of wavelength as a fraction of the rest wavelength • Interpreting the redshift as a Doppler shift:
Measuring Redshift Increasing distance Increasing wavelength
Hubble’s Observations • Hubble made observations of 24 galaxies, measuring their distances and their recessional velocities • He realised that the more distant galaxies were moving away more quickly • In fact, he found that the two are directly proportional • The constant of proportionality is know as the Hubble Constant
Constant of proportionality - The Hubble constant Recessional velocites (Doppler effect) Distances derived using ‘standard candles’
The Hubble Constant • Hubble showed that the recessional velocity of an object (or indeed its redshift) is proportional to its distance from us. • We use to represent the Hubble constant, so can now write: • Measuring the Hubble constant has been a longstanding problem in cosmology!
In reality….. • We are actually quite unsure of the exact geometry of the Universe • Thus the simple relationships stated hold true only for objects at low redshift • For a flat Universe: • But we will stick with the low redshift scenario!
Age of the Universe • From mechanics: • From today: • Thus we can write: • Where t is the time for which the galaxy has been moving away from the earth, ie the time since the Universe began expanding • (Remember that before the big bang, everything existed in a ‘singularity’!)
Fate of the Universe • How the Universe will end is determined by its density • Quite literally how much stuff it contains! • A very dense Universe will fall back in on itself in a ‘big crunch’ • A very sparse Universe will continue expanding forever • Of course, it might be somewhere inbetween these extremes
Big Problem!!!! • We know that only a few percent of the Universe’s mass exists in material that we can see • Over 90% of the contents of the Universe is completely invisible! • We call this mysterious material ‘dark matter’ • We STILL don’t know what it is! • This makes it difficult to determine the density of the Universe
Dark Matter • Unknown compostion • Does not emit or reflect EM radiation • Presence inferred from gravitational effects, e.g. lensing • May be exotic new particles • Or lots of undetected, dark astronomical bodies such as planets or dwarf stars • Its existence is crucial to our current cosmological models!
As for the fate of the Universe……
Critical density: Universe expands forever Less dense: Expansion rate increases More dense: Universe will collapse Accelerating: Dark energy???
Critical Density • We can specify these scenarios via the critical density parameter • The critical density is the density at which the Universe is just closed • If the density of the Universe, is less than , the Universe will expand forever • If is greater than the Universe will stop expanding and collapse back on itself
Critical Density • We can summarise these scenarios via the parameter • If , , expands forever • If , , critical Universe • If , , one day recollapses • We currently believe that the Universe is critical! Seems like quite a coincidence….
How can we measure ? • This is difficult, given that we know we can’t see most of the matter in the Universe! • However, measurements of the CMB can help us • The shape of the CMB power spectrum depends on (amongst other things).
What is being done? • AMiBA • Taiwanese project, based in Hawaii • Testing observations with 7 dishes • Ultimately: 19 dishes? • Will measure the CMB power spectrum
The next big thing…..?
Summary • The Universe started with a Big Bang, and still expands today • We can observe ‘leftover’ radiation - CMB • The velocity of receding galaxies is proportional to their distance away from us • Light from galaxies is ‘redshifted’ • Constant of proportionality is the Hubble Constant • Can use to determine the age of the Universe • The fate of the Universe depends on its density • We currently believe that the Universe is ‘critical’ • This means the it will just continue expanding forever
