- Big Bang Theory – an introduction to the backbone of cosmology
- Testing the Big Bang Model – theories and experiments throughout the years which have supported the idea of a Big Bang
- Cosmic Microwave Background radiation – a snapshot of the early universe which is shedding light on the Big Bang
- WMAP project – a brand new mission designed to unlock the mysteries of the universe
Questions to investigate:
- What is the content of the universe?
- What is the universe’s expansion rate?
- Is it accelerating or decelerating?
- When did the first stars form?
- What is the shape of the universe?
- How old is the universe?
- What will be the fate of the universe?
BIG BANG THEORY
The Big Bang Model is widely accepted as a general description of the formation and evolution of the universe, and is continually tested with observations.
12-14 billion years ago, the diameter of the universe was a few millimeters. It quickly experienced an expansion and cooling which continues today. Remnants of early hot dense matter can still be seen today as cosmic microwave background radiation (CMB). The COBE satellite, launched in 1989, was the first attempt to map Big Bang radiation. The new WMAP satellite, launched in February 2003, has even more resolution and sensitivity, leading to dramatic increases in our understanding of the fundamentals of the early universe.
MISCONCEPTIONS ABOUT BIG BANG THEORY
The Big Bang did NOT occur as an explosion at a single point in space!
Questions beyond the realm of the Big Bang Model include:
- What happened before the Big Bang?
- What ‘caused’ the Big Bang?
- What is the universe expanding into?
Forces described in table below: G = gravity, EM = electromagnetic, WN = weak nuclear, SN = strong nuclear
FOUNDATIONS OF THE BIG BANG MODEL
Big Bang Theory = General Theory of Relativity + Cosmological Principle
Einstein’s General Theory of Relativity (1916) is a generalization of Newton’s Law of Gravity. Gravity is described as a distortion of space and time. The Cosmological Principle is an assumption that matter in the universe is uniformly distributed when averaging over large-scales, and that the distribution of matter is homogeneous and isotropic.
THE COSMOLOGICAL CONSTANT
The first version of relativity predicted expansion. Einstein added the cosmological constant lambda to stop the expansion. After the experimental discovery of expansion, Einstein declared that adding lambda was ‘his greatest mistake’. Was lambda really a mistake? Today there is discussion of reviving the cosmological constant as a term associated with the energy density of the vacuum. Dark energy associated with the cosmological constant could help explain the accelerating expansion and the fate of the universe!
GEOMETRY OF THE UNIVERSE
What determines the shape of the universe?
Average density of matter
Assuming the cosmological principle holds, the universe can have one of three shapes, as shown on the right: closed, open, or flat.
Critical density – 6 H atoms/m^3.
TYPES OF MATTER IN THE UNIVERSE
Radiation » massless and nearly massless particles that move at the speed of light (photons, neutrinos)
Baryonic Matter » ordinary matter (protons, neutrons, electrons)
Dark Matter » exotic non-baryonic matter that interacts weakly with baryonic matter (never directly observed in laboratory)
Dark Energy » mysterious, only type of matter that could cause expansion to accelerate, linked to cosmological constant
How much of each type of matter is there?
TESTING THE BIG BANG MODEL
Theoretical and experimental tests of the Big Bang Theory have been performed since 1929.
- Hubble’s expansion law
- Cosmic microwave background radiation
- COBE and WMAP experiments
All indicate reliability of Big Bang Theory!
EXPANSION OF THE UNIVERSE
In 1929, Hubble found that galaxies outside our own are moving away from us with a speed proportional to their distance from us.
How did Hubble find distances to far-away galaxies?
Stars similar to Cepheid variables were used as distance markers.
Hubble’s Law: velocity = Hubble constant * distance.
Recent estimates of the Hubble constant show that it is between 50 km/sec/Mpc < H < 100 km/sec/Mpc.
COSMIC MICROWAVE BACKGROUND
Cosmic Microwave Background Radiation (CMB) – remnant heat from the Big Bang
1948: CMB predicted by Gamow
1950: CMB predicted by Alpher and Herman
1965: CMB observed as noise in a radio receiver built by Penzias and Wilson
1965: Paper on observations by Penzias and Wilson, paper on cosmological interpretation by Dicke, Peebles, Roll, and Wilkinson
1978: Penzias and Wilson receive Nobel prize in physics
- The CMB has a very uniform temperature across the entire sky of ~2.725 K.
- CMB maps are snapshots from 380,000 years after the Big Bang, the last time that CMB photons directly scattered off matter.
- The COBE and WMAP satellites have provided maps of the CMB that show tiny fluctuations in the temperature, which represents fluctuations in the density of matter in the early universe.
CMB RADIATION: COBE VS. WMAP
WMAP: THE SPACECRAFT
Goal of WMAP – to map the relative CMB temperature over the full sky
- two back-to-back symmetric reflector telescopes focus microwave radiation into receivers
- angular resolution = 0.3o
- sensitivity = 20 mK per 0.3o square pixel
- instrumental artifacts limited to 5 mK per pixel
WMAP: THE ORBIT
L2 orbit – Lissajous orbit about Sun-Earth Lagrange point (position where combined gravitational pull of Earth and Sun equals the centripetal force required to rotate with them), 1.5 million km from Earth.
This special orbit provides the following benefits:
- protection from Earth’s microwave emission and magnetic field
- a stable thermal environment
- the Sun, Earth, and Moon are always behind instrument’s field of view
WMAP: THE SCIENCE
The format of a WMAP map is similar to looking at an oval map of the whole earth.
Microwave radiometers scan ~30% of the sky each day, and the full sky is scanned every six months.
WMAP records five separate frequency bands from 22-90 GHz. The five frequency-dependent maps are compiled into one, and microwave emission from the Milky Way is subtracted out. This procedure is seen on the right.
BEYOND THE BIG BANG MODEL
How do we explain the temperature fluctuations in CMB?
Go BEYOND the Big Bang Theory!
The cosmological principle, an integral part of the Big Bang Model, assumes a uniform distribution of matter on global and local scales. So why are there local structures like galaxies in ‘empty’ space? Big Bang Theory does not answer these questions!
ORIGIN OF STRUCTURE
Why did galaxies form?
Structure grew from the gravitational pull of small fluctuations in the quasi- uniform density of the early universe.
The time sequence at the right shows how galaxies eventually formed beginning with the small clumpings of matter.
This theory was developed by Guth, Linde, Steinhardt, and Albrecht as an extension to the Big Bang Theory.
Proposals of Inflation Theory
- there was a period of extremely rapid expansion just after the Big Bang
- during this time period, the energy density of the universe was dominated by a cosmological constant term
Predictions of Inflation Theory
- the density of the universe is close to critical density
- the geometry of the universe is flat and infinite
- there are equal numbers of hot and cold spots in the CMB radiation
WMAP will directly test these predictions!
PUTTING THE PUZZLE PIECES TOGETHER
WMAP is working to compile a list of properties and characteristics of the universe:
- Abundance of different types of matter
- Expansion (Hubble constant; accelerating, decelerating?)
- Origin of structure
- Shape (open, closed, flat; finite, infinite?)
- Ultimate fate
MATTER IN THE UNIVERSE
Mass discrepancy: the mass inferred for most galaxies is 10 times larger than the mass associated with stars, gas, and dust. This has been confirmed by observations of gravitational lensing, the bending of light predicted by relativity. An example of gravitational lensing is shown in the Hubble photograph at the right.
Dark matter candidates:
- MACHOs (MAssive Compact Halo Objects)
- supermassive black holes
- WIMPs (Weakly Interacting Massive Particles), new forms of matter
EXPANSION AND ORIGIN OF STRUCTURE
WMAP – Hubble constant H0 = 71 km/sec/Mpc (+-5%)
This was measured independently of the usual method using Cepheid variables.
WMAP – Expansion of the universe is accelerating.
‘Cosmological constant matter’ or ‘dark energy’ is critical and accounts for ~73% of the universe’s matter.
WMAP – Stars ignited 200 million years after Big Bang.
Equivalent to the first baby steps in the lifetime of an 80 year old person.
AGE OF THE UNIVERSE
How can we find the age of the universe?
determine the age of the oldest stars by measuring the expansion rate of the universe and extrapolating back to the Big Bang.
Globular clusters – 11-18 billion years old
Measure Hubble constant accurately and extrapolate to find – 12-14 billion years old
WMAP – The universe is 13.7 billion years old.
SHAPE AND FATE OF THE UNIVERSE
WMAP – The universe is flat!
Universal geometry is determined by the struggle between the momentum of expansion and the pull of gravity.
WMAP – The universe will continue to expand forever.
‘Some say the world will end in fire, others say in ice’ – Robert Frost
SUMMARY OF WMAP RESULTS
Big Bang Theory + Inflation Theory + Cosmological Constant Term = New Understanding of the Universe!
Big Bang Theory accurately describes many aspects of the universe’s evolution.
Current theoretical and experimental research is attempting to add to the Big Bang Theory in order to explain observable phenomena.
The WMAP project has recorded a cosmic fingerprint that sheds light on the origin, structure, and fate of the universe.
REFERENCES FOR THIS PAGE:
Wilkinson Microwave Anisotropy Probe Webpage, WMAP Mission, Cosmology 101, http://map.gsfc.nasa.gov
Stephen Weinberg, The First Three Minutes, 1977.
Michael Zeilik and Stephen Gregory, Astronomy and Astrophysics, 1998.