The Strange Universe The Strange Universe Sanjay K. Ghosh Department of Physics Bose Institute Kolkata
Early Efforts Earliest astronomical records : 2000 - 1500 BC Mesopotamian priests : systematic astronomical records • Sumarians, Babylonians, and Egyptians develop astronomy Greece School : 600BC - 200AD • Anaximander, Pythagores, Aristotle, Hipparchus, Ptolmey • Earth is the center of Universe Indian Contribution : 500 – 600 AD Aryabhatta: Sun is the centre of the Solar System. • Varahamihira : earth attract bodies. Arab School : 850 - 1200AD Al-Battani, Al-Sufi, Al-Biruni, Arzachel Andromeda Galaxy, idea of elliptical orbits for planets, Appearance of Milky way
Modern Astronomy 1543AD Polish priest, Nicholas Copernicus: Sun centered solar system (Revival of the idea of Aristarchus of Samos 200BC) 1572AD Danish Astronomer,Tycho Brahe – Last naked-eye astronomer :exploding star- NOVA, Comet orbit Spectacle makerHans Lippershey (1570 -1619): Assembled first telescope 1610AD Italian, Galileo Galilei : Use of telescope 1620AD John Kepler : Kepler’s Law More and better observational data
Distance measurement known from 1700 Works only for nearby star Need for Standard Candles to measure larger distances
Standard Candles : Cepheid Variables giant yellow star pulsing regularly by expanding and contracting fairly tight correlation between period of variability and absolute stellar luminosity (total light per sec) Luminosity related to apparent brightness (light received/area/sec.) and distance (from parallax method) Period – Luminosity Law (Henrietta Swan Leavittin 1912)
Harvard College Observatory Edward Charles Pickering Director of HCO (1877-1919) "Pickering's Harem" or, Harvard Computers. A photograph such as this shows bright stars as larger disks than fainter stars
The synthesis of helium At temperatures above 1010 K, any deuteron formed from a neutron—proton collision was quickly disrupted by a collision because the thermal energies involved often exceeded the 2.2 MeV binding energy of the deuteron. The only nuclei existing at these temperatures were single protons and neutrons. In normal circumstances a neutron beta decays with a mean life of about 15 minutes to a proton, an electron and an anti-neutrino, At high temperature and density, neutrons can be transformed to protons, and protons can be transformed to neutrons in collisions involving thermal neutrinos, anti-neutrinos,electrons & positrons Heavier neutron – more energy needed for creation – no. of neutron < no.of proton
n and p ratio decreased with decreasing T – expanding Universe Ratio became about 1/5 just below 1010 K – further decrease due neutron decay After a few minutes, when n decay had reduced the n-p ratio to about 1/7, the universe was cool enough for a sequence of two-body reactions - bound states of n and p. At about 109 K deuteron nuclei began to be present in significant amounts as n-p radiative capture, n + p d + , competed successfully with deuteron photodisintegration, + d n + p. Capture of neutrons and protons by deuterons led to the formation of tritons and helium-3. These nuclei in turn captured p and n to form helium-4. Since helium-4 is by far the most stable nucleus in this region of the periodic table, nearly all the neutrons that existed when the temperature was 109 K were converted into helium-4. Moreover, the absence of stable nuclei with mass 5 and 8 prevented the formation of more massive nuclei, apart from small amounts of lithium-7.
Problem with BBN • Input for BBN – baryon to photon ratio • 5.89 x 10-10 < η < 6.39 x 10-10 baryons/photon • BBN provided the raw material for the first stars • Gravitational Contraction Gravity is the driving force behind stellar evolution. Most importantly it leads to the compression of matter and thence to the formation of stars. It leads to the conditions where nuclear forces play a constructive role in thermonuclear fusion. The transformation of hydrogen to helium in the hot compressed centres of stars is often followed by a further compression and the transformation of helium into more massive elements such as carbon, oxygen and iron, the star dust out of which we are all made.
Burning Chain H He C O Ne Si Fe
Arthur Eddington (1924) : Mass-Luminosity relationship Outward radiation pressure = inward gravity Fritz Zwicky (1933) : Measured velocity of eight galaxies in COMA cluster Mass/Luminosity is much larger than expected from mass-luminosity relation Vera Rubin (1975) most stars in spiral galaxy orbit roughly at the same speed Presence of DARK MATTER in the galaxies
The average speed of galaxies within a cluster depends on the total mass of the cluster, since each galaxy is attracted by the gravity of all the others. • From the observed speeds of galaxies moving within the Coma cluster, Zwicky calculated its total mass. • added up all the light from the galaxies in the cluster and used it to calculate the mass in the form of luminous stars. • mass of the cluster based on the speed of its galaxies was about ten times more than the mass of the cluster based on its total light output. • Coma cluster must contain an enormous quantity of unseen matter, with enough gravity to keep the rapidly moving galaxies from flying apart • Dark Matter
Darkmatter: What are they • DEAD stars ???? • Primordial black holes ??? • Weakly Interacting massive particles???
Hubble’s Law (1929) : Expanding Universe - Cepheid variables – Leavittin’s relation • Recessional Velocity = Hubble's constant times distance • V = Ho D where • V is the observed velocity of the galaxy away from us, usually in km/sec • H is Hubble's "constant", in km/sec/Mpc • D is the distance to the galaxy in Mpc V is related to red shift
Gravitational Red Shift: A heavy object is denoted by a deformation of space represented by the funnel. As light leaves the vicinity of this object it is shifted towards the red: for a sufficiently compact and massive object a blue laser on the surface will be seen as red in outer space.
The light emitted by stars and gas in distant galaxies has been stretched to longer wavelengths during its journey to Earth. This shift in wavelength is given by the redshift, z = (λobs – λ0)/λ0, where λobs is the wavelength we see on Earth and λ0 is the wavelength of the emitted light. • Prime methods for measuring extragalactic distances - “standard candles” such as Cepheid variable stars. • The distance to a Cepheid - first measure its period to obtain the luminosity • then compare this with the observed intensity to calculate the distance. • Thus, redshifts and distances to objects moving in the “Hubble flow” (the region beyond the gravitational influence of our local group of galaxies) have been charted, revealing the Hubble law: d = (cz/H0), where c is the speed of light and H0 = 72 ± 8 km s-1 per megaparsec (Mpc) is the Hubble constant (1 Mpc is equal to 3.26 million light-years). Going the distance Although Cepheid variable stars have proved extremely valuable as standard candles in astronomy for many years, they are not bright enough to be used at high redshifts. However, astronomers have found a very special type of supernova to take their place.
Can one explore farther : New Candles Type IA supernova Binary system of white dwarf and red giant accretion onto the white dwarf reaches Chandrasekhar Limit Gigantic thermonuclear reaction
Mid 1990 High z Supernova search (Mt. Stromlo and Siding spring Observatory,Australia) International Supernova Cosmology Project (LABL, USA) 1998 - Recorded 100 or so supernova Observations of supernovae can be used to chart the history of the cosmic expansion. (a) The distance to a type 1a supernova is obtained from its luminosity, which is calibrated by its light curve and spectrum, and its observed intensity. (b) Meanwhile, the expansion of the universe shifts features in the supernova spectrum to longer wavelengths by a factor characterized by the redshift. (c) By plotting distance versus redshift for a large number of supernovae, we can chart how the universe has expanded over time.
orange circles -data points along with the theoretical prediction: a universe with 30% matter and 70% cosmological constant (blue). universe with 30% matter and spatial curvature (red dashed) 100% matter (purple dashed). Green- no acceleration or deceleration
(Rainer Sachs & Art Wolfe) Integrated Sachs-Wolfe (ISW) Effect: Gravitational potential wells of dense and overdense regions in the universe have been stretched and made shallower over time Influence of repulsive gravity (or acceleration)
The Sloan Digital Sky Survey (SDSS) identifies Galaxy Concentrations and determines their positions on the sky. • The Wilkinson Microwave Anisotropy Probe (WMAP) measures the angular pattern of energies of the Cosmic Microwave Background Radiation (CMBR) [reds,yellows, greens, blues, purples, in order of increasing energy].
Position of the peak in this spectrum depends on the geometry of the universe. Recent observations confirm that the peak occurs at the position predicted for a flat universe (blue). In an open universe the peak would be to the left (red), and in a closed universe it would be on the right (green).
WMAP • (Wilkinson Microwave Anisotropy Probe) • Universe is 13.7 billion years old (±1%) • First stars ignited 200 million years after the Big Bang • Content of the Universe: 4% Atoms, 23% Cold Dark Matter, 73% Dark Energy • Expansion rate (Hubble constant): H0= 71 km/sec/Mpc (±5%)
Evolution of the concept Einstein : Cosmological Constant General theory of relativity (1916) Universe either expands or contracts For Static Universe Cosmological Constant Hubble’s Theory (1929) • Expanding Universe Cosmological Constant dropped
Attempts for revival • 1960 : Vacuum energy of particles and fields should generate • 1980 : Theory of Inflation Early Universe goes through brief period of accelerated exponential expansion -ve pressure drives the expansion Inflaton
Candidates : ( pDark = w eDark ) • Cosmological Constant : Static (w = -1) • Quintessence : Dynamic (w > -1) • Other Vacuum Energy (w < -1) • Modification of GTR
Dark Energy • CDM : Dust like equation of state Pressure p=0 Energy density e > 0 • Dark energy : p=w e; w < 0 (Ideally w= -1) +ve energy -ve pressure
Dark Energy (a) emits no light (b) it has large –ve pressure (c) does not show its presence in galaxies and cluster of galaxies, it must be smoothly distributed
e c~ 10-47 GeV4 , So for DE ~ 0.7, eDE ~ 10-48 GeV4 Natural Units Natural Explanation : Vacuum energy density with correct equation of state Difficulties : higher energy scales Planck era : ~ 1077 GeV4 GUT : ~ 1064 GeV4 Electroweak : ~ 108 GeV4 QCD : ~ 10-4 GeV4 Puzzle Why eDE is so small ???