2Overview The Big Bang The Expanding Universe The Fate of the Universe Formation of the UniverseThe Cosmic Microwave BackgroundThe COBE experimentThe Expanding UniverseRedshiftHubble’s LawThe age of the UniverseThe Fate of the UniverseThe critical densityBig crunch or big chill?
3Cosmology: ‘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.’
4The 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
7The Big Bang Not really an ‘explosion’ Universe expanded rapidly as a wholeUniverse is still expanding today as a result of the Big BangMatter was created in the form of tiny particles (protons, neutrons, electrons)Too hot for normal ‘stuff’ to form (eg atoms, molecules)
10Formation 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
11Discovery of the CMBPenzias and Wilson record excess noise when observing the galaxySoon identified as the CMBIsotropic to 1 part in 100,000 - recognised as evidence for the big bang
12What is the CMB like?Can observe the CMB photons today, 13.7Gyr after the Big BangRadiation 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 equilibriumSo what does it look like?
13The Cosmic Background Explorer (COBE) First satellite dedicated to CMB researchLaunched by NASA in 1989Measured the spectrum of the CMBAlso measured temperature fluctuationsScientists won the Nobel Prize in 2007! (15 years after releasing their results)
14A perfect blackbodyRemnant heat of the creation of the Universe
15Observe ‘blank’ sky with a radio telescope (eg COBE) Rather than darkness, see Uniform, high-energy glowHigh sensitivity measurements reveal......
17Tiny temperature differences When the CMB photons ‘escaped’, structures were starting to formThese structures have now become galaxiesThe structure formation processes have affected the CMB and we see the imprint as ‘hot’ and ‘cold’ spotsVery difficult to measure!
18What does the CMB tell us? Measure the strength of the temperature differences on different scales, eg:
19What does the CMB tell us? Measure the strength of the temperature differences on different scales, eg:
21ParametersThe function on the previous slide is complex and involves many parameters including:The Hubble constantThe density of the UniverseThe curvature of the UniverseThe age of the UniverseAnd more…..We can constrain some of the big questions in cosmology by observing the CMBWe will look at some of these in more detail later
23Edwin HubbleEdwin Hubble discovered objects beyond the Milky Way - AndromedaHe realised that these objects are all moving away from usBy the cosmological principle, we see that all objects are moving away from each other
25RedshiftThe light from a galaxy which is moving away from us will be Doppler ShiftedAs the object is moving away from us, the light will be shifted towards the red end of the spectrumWe refer to this as the cosmological redshift for objects moving along with the general expansion of the Universe
29Hubble’s Observations Hubble made observations of 24 galaxies, measuring their distances and their recessional velocitiesHe realised that the more distant galaxies were moving away more quicklyIn fact, he found that the two are directly proportionalThe constant of proportionality is know as the Hubble Constant
30Constant of proportionality - The Hubble constant Recessional velocites (Doppler effect)Distances derived using ‘standard candles’
31The Hubble ConstantHubble 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!
32In reality…..We are actually quite unsure of the exact geometry of the UniverseThus the simple relationships stated hold true only for objects at low redshiftFor a flat Universe:But we will stick with the low redshift scenario!
33Age 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’!)
34Fate of the UniverseHow the Universe will end is determined by its densityQuite 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 foreverOf course, it might be somewhere inbetween these extremes
35Big Problem!!!!We know that only a few percent of the Universe’s mass exists in material that we can seeOver 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
36Dark Matter Unknown compostion Does not emit or reflect EM radiation Presence inferred from gravitational effects, e.g. lensingMay be exotic new particlesOr lots of undetected, dark astronomical bodies such as planets or dwarf starsIts existence is crucial to our current cosmological models!
38Critical density: Universe expands forever Less dense: Expansion rate increasesMore dense: Universe will collapseAccelerating: Dark energy???
39Critical DensityWe can specify these scenarios via the critical density parameterThe critical density is the density at which the Universe is just closedIf the density of the Universe, is less than , the Universe will expand foreverIf is greater than the Universe will stop expanding and collapse back on itself
40Critical Density We can summarise these scenarios via the parameter If , , expands foreverIf , , critical UniverseIf , , one day recollapsesWe currently believe that the Universe is critical! Seems like quite a coincidence….
41How 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 usThe shape of the CMB power spectrum depends on (amongst other things).
45Summary The Universe started with a Big Bang, and still expands today We can observe ‘leftover’ radiation - CMBThe velocity of receding galaxies is proportional to their distance away from usLight from galaxies is ‘redshifted’Constant of proportionality is the Hubble ConstantCan use to determine the age of the UniverseThe fate of the Universe depends on its densityWe currently believe that the Universe is ‘critical’This means the it will just continue expanding forever