 Cosmic Background Explorer (COBE) |
Wilkinson Microwave Anisotropy Probe (WMAP). |
T=2.725 K and distortions on the order of 0.001 %!
Cosmic Background Explorer (COBE) |
Wilkinson Microwave Anisotropy Probe (WMAP). |
T=2.725 K and distortions on the order of 0.001 %!
The Big Bang theory for the Universe is in reasonably good shape.
The Universe is expanding, is bathed in a
low temperature sea of background
radiation (the CMBR), and has a
chemical
composition (90 %hydrogen and 10 % helium) all of which are
naturally understood in terms of the Big Bang theory.
There are problems and ultimate questions for sure,
but currently the Big Bang is rather secure.
A potential problem is that the Universe is thought to be roughly 13.7 billion years old based on the Hubble constant and some simple assumptions. The ages of the oldest stars in our Galaxy (the Milky Way galaxy) are conservatively inferred to be 11-18 billion years (based on studies of globular clusters). The large uncertainty in the age estimate may hide a problem. If it turns out that stars are older than 13.7 billion years, then our current modeling of the Universe is unacceptable and will have to be modified.
We represent models for the Universe by either considering the Hubble Law or the scale factor R(t) for the Universe.
Hubble diagrams for open, closed, and flat universes. Closed universes have the highest recession speeds at great distances (they have suffered the largest slow-downs. The parameter Omega > 1 means closed universe and Omega < 1 means open universe. Flat universes have Omega = 1. |
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Scale Factor R(t) plots for open, closed, and flat universes. R(t) measures the size of the universe. It compares the size of the universe at different times. The blue curve represents a closed universe. The parameter Omega > 1 means closed universe and Omega < 1 means open universe. Flat universes have Omega = 1. |
The principal differences between the models is in how the Universe winds-up its evolution. The early times are fairly similar. This simplifies our discussion.
The Planck era when all four forces of the Universe -- the gravitational, electro-magnetic, strong (nuclear), and weak forces -- may have been unified. This is the era of quantum gravity -- the time when a theory which encompasses both quantum mechanics and gravity needs to be used. This phase awaits the development of a theory of quantum gravity. The size of the current Universe is less than 10-50 centimeters in the Planck era.
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Era of the Grand Unified Theories (GUTs) when gravity separates from the other three forces (i.e., gravity becomes an interaction distinct from the other forces). Inflation begins at the end of this stage -- the inflation is driven by a repulsive force -- leading to a rapid, exponential growth of the Universe. See Inflation . |
Inflation has acted by the beginning of this stage -- the Universe inflated by perhaps a factor of 101012; the size of our current Universe increased from 10-50 centimeters to roughly the size of a grapefruit during inflation. Amazingly, even a region as small as the Planck length (~0.000000000000000000000000000000001 cm) expands enough during inflation to encompass the currently observable Universe. After inflation, the Universe enters the Quark (Hadron) Phase where it consists of an electron-quark soup. At around 10-10 seconds, the weak and electromagnetic forces split, and we finally have our 4 known forces.
Before t ~ 10-4 second, T was greater than 1013 K, and matter and radiation were in equilibrium. That is, the creation and annihilation of material were going full-bore. What does this mean? We already talked a little about pair-formation in vacuum where we noted that matter/anti-matter pairs would appear spontaneously and not upset the energy balance of the Universe, if the pairs disappeared (annihilated each other) quickly enough. Such matter/anti-matter pairs, in a sense, are not real. They are referred to as virtual pairs of particles.
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Left: schematic picture of pair production and annihilation. Above: Bubble Chamber photograph showing production of an electron-position (anti-electron) pair in a magnetic field. The gamma-ray comes in from the left hand side and creates the pair (the two spirals). A bubble chamber is a vessel filled with transparent unstable liquid (for example, mixtures of hydrogen and neon have been used). When a particle passes through the liquid, the heat generated boils the liquid leaving a trail of bubbles (the tracks). The bubble chamber is an early particle detector. |
The more massive the particle, the larger the energy required to produce them. (Recall Einstein's famous relation
---> mass and energy measure the same quantity, they are just different forms of it.) Below T ~ 1013 K, it is not hot enough to make protons and neutrons. This temperature cut-off forms the Temperature Threshold for proton/anti-proton and neutron/anti-neutron pair production. For electron/positron pair production, the corresponding Temperature Threshold is 6x109 K, below the threshold for proton/anti-proton pair production because electrons are around 1/1,836-th the mass of a proton.
Particle production can occur as long as the radiation is energetic enough (the Universe is hot enough). The early Universe is plenty hot enough and creation of particle pairs via the annihilation of photons occurs easily. Whether matter survives till today, however, relies on whether the matter and anti-matter can be kept apart. If matter and anti-mattre collide, they will annihilate (destroy matter) and produce radiation (create photons).
At this stage, the Universe is primarily light particles like electrons, neutrinos, and muons, and their anti-matter twins.
Around t = 1 seconds, the density and T no longer high enough to maintain strong coupling between neutrinos and other forms of matter ===> the Universe becomes transparent to neutrinos (this de-coupling is in the same sense as later found when the photons de-couple, the Era of Recombination). If we ever become technologically advanced enough to detect low energy neutrinos, then we, in principle, would be able to detect this Cosmic Neutrino Background (CNB) and so probe the Universe when it was ~ 1 second old!
An amusing note is that our understanding of nuclear reactions is much better for the conditions found in the Big Bang than for the conditions found in the interiors of stars!! The Big Bang is hotter than the interiors of stars. Terrestrial experimenters actually perform experiments at temperatures closer to Big Bang temperatures than to the low temperatures found in say the core of the Sun.
The Universe does not not make elements more massive than helium (the second simplest element). This is due to the fact that the elements slightly more massive than helium are unstable. For example, stars circumvent this bottleneck using what is referred to as the triple alpha process. To make carbon in stars, the following events occur,
The second step must occur very soon after step 1 is completed. Berylium is very unstable; it will decay back into the 2 helium nuclei with a half-life on the order of 2.6x10-16 seconds! In stars because of their large densities, the capture of the third helium atom can happen quickly enough and carbon is produced. In the early Universe, the density is too low for the Berylium to capture the third helium nucleus before it decays and so this is not a viable way to get around the bottleneck. Other ways are also unsuccessful and element production essentially ends at helium with only small amounts of elements less massive than Berylium produced.
The Universe becomes cool enough (T ~ 3,000 K, redshift z ~ 1,000) and rarefied enough for electrons and protons to form neutral hydrogen atoms. Neutral hydrogen atoms are not efficient absorbers of radiation and the Universe becomes transparent to radiation, the matter and radiation decoupled. Before this time, the Universe was opaque to radiation, that is, the Universe was "foggy" in that light would not travel very far which meant that your view of the Universe was limited to only what was nearby.
After the neutral hydrogen formed, which is known as the Era of Recombination (De-Coupling), we can get unimpeded view of the distant Universe. This is the phase of the Universe which we observe when we look at the CMBR. Note that we cannot see any earlier in the history of the Universe in visible light than the Era of Recombination.
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As we look deeper and deeper into space, we see further and further back into time. At the Era of Recombination, the Universe becomes opaque to visible light and our view back into time stops. To see further back in time, we must look for things that can move freely through the Universe closer to the beginning. Neutrinos and gravity waves are possible candidates for study. |
Protogalaxies begin to form. During the 1980s this was annoying because the clumps of matter and the clumps of the clumps of normal matter needed to have galaxies at this early time would strongly distort the CMBR leading to large temperature distortions around the sky that were not seen. To form galaxies and clusters of galaxies and the walls and voids by the attraction of gravity requires a fairly long time. This means that by the time of recombination, the process must have been fairly well along and the ovedensities at CMBR formation would already be large.
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The way out of this problem was to recognize that normal matter (which interacts strongly with electromagnetic radiation) does not dominate the Universe. Dark Matter which does not interact strongly with light dominates normal matter. Dark matter can easily clump and remain fairly invisible in its effects on the CMBR. Consequently, dark matter is able to start clumping strongly and forming structure and not distort the CMBR! So, not only is dark matter is needed to explain rotation curves, gravitational lensing, and the hot gas is galaxy clusters, it is needed to produce the structure we see in the Universe! |
Currently, we may have already detected the largest structures in the Universe; the size of which are consistent with the imperfections seen in the CMBR.
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Solar System, planets, life, ...
