Recent advances in physics and astronomy --- our current understanding of the Universe Lecture 4: Big Bang, the origin of the Universe April 23th, 2003
Universe to Cherokee • Cherokee, now a famous brand for many merchandise, is an American Indian tribe who lived in South Carolina for centuries. • The legends abound in the Cherokee culture believed the Universe was made up of three separate worlds: the Upper World, the Lower World, and This World and we are living in “This World”. • Four directions, East, North, South and West are each associating with a color (red, blue white and black). This rather primitive view of our Universe, like those advocated by European astronomers some 500 hundreds year ago ----(that universe is composed of spheres within spheres) ignores the time flow and regards the universe being static.
Olbers’s Paradox Why is the night sky dark? Why isn't the night sky as uniformly bright as the surface of the Sun? If the Universe has infinitely many stars, then it should be. After all, if you move the Sun twice as far away from us, we will intercept one quarter as many photons, but the Sun will subtend one quarter of the angular area. So the area intensity remains constant. With infinitely many stars, every angular element of the sky should have a star, and the entire heavens should be as bright as the sun. This is Olbers' paradox.
So… what is the explanation? • There's too much dust to see the distant stars. • Wrong! Dust will be heated and radiate light! • The Universe has only a finite number of stars. • Mmmmm! If number of stars are finite, so is the size of Universe? • The distribution of stars is not uniform. So, for example, there could be an infinity of stars, but they hide behind one another so that only a finite angular area is subtended by them. • Why is earth so special that stars align in a such a way? • The Universe is expanding, and distant stars leave us faster, so photons from distant stars are harder to reach us. ( Bingo!) • The Universe is young. Distant light hasn't even reached us yet. • (Bingo!)
The Cosmological principle • To study the universe as a whole, we need • General Relativity describes space and time, and its relation with the matter contained in it: "Matter tells space how to curve, and space tells matter how to move." • an assumption about how matter is distributed in the universe. Cosmology principle: the matter in the universe is homogeneous and isotropic when averaged over very large scales.
The Birth of Big Bang Theory • In 1927, Georges Lemaître (a Belgian priest) firstly proposed that the universe began with the explosion of a primeval atom. • Edwin Hubble, in 1929 found found that distant galaxies in every direction are going away from us with speeds proportional to their distance. • Top three reasons (and tests) to believe big bang cosmology • Hubble Expansion • Cosmic Microwave Background • Big Bang Nucleosynthesis
Geometry of Universe The geometry of the universe strongly depends on how much matter is contained within it. There exist a critical energy density, above which, our universe is “closed”, and below which, our universe in “open”.
The curvature of the Universe Orange: closed Green: flat Blue: open, but decelerating Red: open and accelerating ( the likely scenario for our own Universe)
An expanding Universe, how? • Assuming we are living in a 2-D world. For example, on a surface of a balloon. • An expanding universe can be think of the processing of airing the balloon. • There is no center, i.e. no special point on the surface. Each point can be regarded as the center. • The distances (measured on the surface) between any two points will double all at the same time.
Hubble’s law Hubble’s law: Galaxies are receding from each other at speeds between 18 million km/hr and 72 million km/hr. corresponding to length scales of 200 million light years and 800 million light years. • Hubble constant value: Ho= 71 km/sec/Mpc (with a margin of error of about 5%)
Test of Big Bang • The expansion of the universe • Edwin Hubble's 1929 observation that galaxies were generally receding from us provided the first clue that the Big Bang theory might be right. • The cosmic microwave background (CMB) radiation • The early universe should have been very hot. The cosmic microwave background radiation is the remnant heat leftover from the Big Bang. • The abundance of the light elements H, He, Li • The Big Bang theory predicts that these light elements should have been fused from protons and neutrons in the first few minutes after the Big Bang (next lecture).
Signal or Noise? • Predicted by George Gamov in 1948. • Observed by Robert Wilson and Arno Penzias in 1965. 1978 Nobel Prize winners
Wavelength (m) Frequency (Hz) Energy (J) Radio > 1 x 10-1 < 3 x 109 < 2 x 10-24 Microwave 1 x 10-3 - 1 x 10-1 3 x 109 - 3 x 1011 2 x 10-24- 2 x 10-22 Infrared 7 x 10-7 - 1 x 10-3 3 x 1011 - 4 x 1014 2 x 10-22 - 3 x 10-19 Optical 4 x 10-7 - 7 x 10-7 4 x 1014 - 7.5 x 1014 3 x 10-19 - 5 x 10-19 UV 1 x 10-8 - 4 x 10-7 7.5 x 1014 - 3 x 1016 5 x 10-19 - 2 x 10-17 X-ray 1 x 10-11 - 1 x 10-8 3 x 1016 - 3 x 1019 2 x 10-17 - 2 x 10-14 Gamma-ray < 1 x 10-11 > 3 x 1019 > 2 x 10-14 EM radiation and its classification
COBE Cosmic Background Explorer (COBE)Three experiments: • Far Infrared Absolute Spectrophotometer (FIRAS): compare the spectrum of the cosmic microwave background radiation with a precise blackbody. • Differential Microwave Radiometer (DMR) : map the cosmic radiation sensitively, searching for anisotropy. • Diffuse Infrared Background Experiment (DIRBE): search for the cosmic infrared background radiation in the wavelength range of 1.25 to 240 microns. Launched on Nov. 18th, 1989.
Results from COBE • FIRAS - The cosmic microwave background (CMB) spectrum is that of a nearly perfect blackbody with a temperature of 2.725 +/- 0.002 K. Agrees with the hot Big Bang theory. All of the radiant energy of the Universe was released within the first year after the Big Bang. • DMR - The CMB was found to have intrinsic "anisotropy" for the first time, at a level of a part in 100,000. These tiny variations in the intensity of the CMB over the sky show how matter and energy was distributed when the Universe was still very young. • DIRBE - Infrared absolute sky brightness maps in the wavelength range 1.25 to 240 microns were obtained to carry out a search for the cosmic infrared background (CIB). Subsequent analyses have yielded detections of the CIB in the near-infrared DIRBE sky maps. The CIB represents a "core sample" of the Universe; it contains the cumulative emissions of stars and galaxies dating back to the epoch when these objects first began to form.
The anisotropy from COBE • In 1992, (COBE) detected tiny fluctuations in the cosmic microwave background. It found, for example, one part of the sky has a temperature of 2.7251 Kelvin (degrees above absolute zero), while another part of the sky has a temperature of 2.7249 Kelvin. • These fluctuations are related to fluctuations in the density of matter in the early universe and thus carry information about the initial conditions for the formation of cosmic structures such as galaxies, clusters, and voids. • COBE had an angular resolution of 7 degrees across the sky, 14 times larger than the Moon's apparent size. This made COBE sensitive only to broad fluctuations of large size.
Why fluctuations? • Gravity can enhance the tiny fluctuations seen in the early universe, by can not produce these fluctuations. • Two popular ideas are: • Inflation • Topological Defects • Different predictions about the properties of the cosmic microwave background fluctuations. For example, the inflationary theory predicts that the largest temperature fluctuations should have an angular scale of one degree, while the defect models predict a smaller characteristic scale.
From COBE to WMAP • Need better angular resolution. The Wilkinson Microwave Anisotropy Probe (WMAP). • WMAP is able to map the relative CMB temperature over the full sky with an angular resolution of at least 0.3°, a sensitivity of 20 µK per 0.3° square pixel, with systematic artifacts limited to 5 µK per pixel.
The WMAP spacecraft Launched to L2 point on June 30, 2001
Results from WMAP • the first generation of stars first ignited only 200 million years after the Big Bang, much earlier than many scientists had expected. • the age of the Universe is decided to be 13.7 billion years old, with a remarkably small 1% margin of error. • The Inflation theory about early universe is likely true. • The contents of the Universe include 4% atoms (ordinary matter), 23% of an unknown type of dark matter, and 73% of a mysterious dark energy.
Beyond Big Bang • Why is the universe so uniform on the largest length scales? • Why is the physical scale of the universe so much larger than the fundamental scale of gravity, the Planck length, which is one billionth of one trillionth of the size of an atomic nucleus? • Why are there so many photons in the universe? • What physical process produced the initial fluctuations in the density of matter?
Inflation and its prediction • That the density of the universe is close to the critical density, and thus the geometry of the universe is flat. • That the fluctuations in the primordial density in the early universe had the same amplitude on all physical scales. • That there should be, on average, equal numbers of hot and cold spots in the fluctuations of the cosmic microwave background temperature.
References • Webpages: 1) A primer of cosmology http://map.gsfc.nasa.gov/m_uni.html 2) A tutorial about universe http://imagine.gsfc.nasa.gov/index.html • Books: 1) Before the beginning by Martin J. Rees 2) The first three minutes by Steven Weinberg 3) A brief History of time: From the Big Bang to Black holes by Stephen Hawking 4) The Inflationary Universe: The Quest for a New Theory of Cosmic Origins by Alan Guth 5) Cosmos by Carl Sagan