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By the end of this section, you will be able to:
  • Describe the evolution of the early universe in terms of the four fundamental forces
  • Use the concept of gravitational lensing to explain astronomical phenomena
  • Provide evidence of the Big Bang in terms of cosmic background radiation
  • Distinguish between dark matter and dark energy

In the previous section, we discussed the structure and dynamics of universe. In particular, the universe appears to be expanding and even accelerating. But what was the universe like at the beginning of time? In this section, we discuss what evidence scientists have been able to gather about the early universe and its evolution to present time.

The early universe

Before the short period of cosmic inflation, cosmologists believe that all matter in the universe was squeezed into a space much smaller than an atom. Cosmologists further believe that the universe was extremely dense and hot, and interactions between particles were governed by a single force. In other words, the four fundamental forces (strong nuclear, electromagnetic, weak nuclear, and gravitational) merge into one at these energies ( [link] ). How and why this “unity” breaks down at lower energies is an important unsolved problem in physics.

Figure shows a timeline. At 10 to the power minus 43 seconds after big bang, the line splits into two. One branch is gravitational force. The other moves ahead and further splits into two at 10 to the power minus 35 seconds. From here, one branch is strong nuclear force. The other splits into two at 10 to the power minus 12 seconds. The two branches are labeled electromagnetic force and weak nuclear force. The particle energy and the temperature of the universe at the time of the first split are: 10 to the power 19 GeV and 10 to the power 32 K respectively. At the second split, they are: 10 to the power 14 GeV and 10 to the power 27 K respectively. At the third split, they are 100 GeV and 10 to the power 15 K respectively. All four lines continue till they reach the values: 5 into 10 to the power 17 seconds, 10 to the power minus 4 eV and 3 K.
The separation of the four fundamental forces in the early universe.

Scientific models of the early universe are highly speculative. [link] shows a sketch of one possible timeline of events.

Figure shows a timeline. Inflation starts at 10 to the power minus 43 seconds after big bang, at a temperature of 10 to the power  32 K and an energy of 10 to the power 19 GeV. Inflation ends at 10 to the power minus 35s, 10 to the power 27 K and 10 to the power 15 GeV. This is followed by Age of leptons: quarks, muons, taus, gluons and photons. Protons are formed at 10 to the power minus 6 s, 10 to the power 13 K and 0.1 GeV. This is followed by the age of nucleons: quarks, protons, muons, neutrons, taus, electrons, mesons, photons. Nuclear fusion begins at 225 s, 10 to the power 11 K and 10 to the power minus 4 GeV. This is followed by the age of nucleo synthesis: protons, He, electrons, photons. Nuclear fusion ends at 1000 years, 100,000 K and 10 to the power minus 8 GeV. This is followed by the age of ions: protons, positrons, He, electrons, photons. Cosmic microwave background is at 3000 years, 60,000 K and 5 into 10 to the power minus 9 GeV. This is followed by age of atoms. First stars and galaxies are formed at 300,000 years, 3000 K and 3 into 10 to the power minus 10 GeV. This is followed by the age of stars and galaxies. Today the temperature is 2.7 K and the energy is 2.3 into 10 to the power minus 13 GeV.
An approximate timeline for the evolution of the universe from the Big Bang to the present.
  1. Big Bang ( t < 1 0 43 s ) : The current laws of physics break down. At the end of the initial Big Bang event, the temperature of the universe is approximately T = 1 0 32 K .
  2. Inflationary phase ( t = 1 0 43 to 1 0 35 s ) : The universe expands exponentially, and gravity separates from the other forces. The universe cools to approximately T = 1 0 27 K .
  3. Age of leptons ( t = 1 0 35 to 1 0 6 s ) : As the universe continues to expand, the strong nuclear force separates from the electromagnetic and weak nuclear forces (or electroweak force). Soon after, the weak nuclear force separates from the electromagnetic force. The universe is a hot soup of quarks, leptons, photons, and other particles.
  4. Age of nucleons ( t = 1 0 6 to 225 s ) : The universe consists of leptons and hadrons (such as protons, neutrons, and mesons) in thermal equilibrium. Pair production and pair annihilation occurs with equal ease, so photons remain in thermal equilibrium:
    γ + γ e + e + γ + γ p + p γ + γ n + n .

    The number of protons is approximately equal to the number of neutrons through interactions with neutrinos:
    ν e + n e + p ν e + p e + + n.

    The temperature of the universe settles to approximately 1 0 11 K —much too cool for the continued production of nucleon-antinucleon pairs. The numbers of protons and neutrons begin to dominate over their anti-particles, so proton-antiproton ( p p ) and neutron-antineutron ( n n ) annihilations decline. Deuterons (proton-neutron pairs) begin to form.
  5. Age of nucleosynthesis ( t = 225 s to 1000 years): As the universe continues to expand, deuterons react with protons and neutrons to form larger nuclei; these larger nuclei react with protons and neutrons to form still larger nuclei. At the end of this period, about 1/4 of the mass of the universe is helium. (This explains the current amount of helium in the universe.) Photons lack the energy to continue electron-positron production, so electrons and positrons annihilate each other to photons only.
  6. Age of ions ( t = 1000 to 3000 years): The universe is hot enough to ionize any atoms formed. The universe consists of electrons, positrons, protons, light nuclei, and photons.
  7. Age of atoms ( t = 3000 to 300,000 years): The universe cools below 10 5 K and atoms form. Photons do not interact strongly with neutral atoms, so they “decouple” (separate) from atoms. These photons constitute the cosmic microwave background radiation to be discussed later.
  8. Age of stars and galaxies ( t = 300,000 years to present): The atoms and particles are pulled together by gravity and form large lumps. The atoms and particles in stars undergo nuclear fusion reaction.

Questions & Answers

What is motion
Izevbogie Reply
Anything which changes itself with respect to time or surrounding
and what's time? is time everywhere same
how can u say that
do u know about black hole
Not so more
Radioactive substance
These substance create harmful radiation like alpha particle radiation, beta particle radiation, gamma particle radiation
But ask anything changes itself with respect to time or surrounding A Not any harmful radiation
explain cavendish experiment to determine the value of gravitational concept.
Celine Reply
For the question about the scuba instructor's head above the pool, how did you arrive at this answer? What is the process?
Evan Reply
as a free falling object increases speed what is happening to the acceleration
Success Reply
of course g is constant
acceleration also inc
which paper will be subjective and which one objective
normal distributiin of errors report
normal distribution of errors
photo electrons doesn't emmit when electrons are free to move on surface of metal why?
Rafi Reply
What would be the minimum work function of a metal have to be for visible light(400-700)nm to ejected photoelectrons?
Mohammed Reply
give any fix value to wave length
40 cm into change mm
Arhaan Reply
40cm=40.0×10^-2m =400.0×10^-3m =400mm. that cap(^) I have used above is to the power.
i.e. 10to the power -2 in the first line and 10 to the power -3 in the the second line.
there is mistake in my first msg correction is 40cm=40.0×10^-2m =400.0×10^-3m =400mm. sorry for the mistake friends.
40cm=40.0×10^-2m =400.0×10^-3m =400mm.
this msg is out of mistake. sorry friends​.
what is physics?
sisay Reply
why we have physics
Anil Reply
because is the study of mater and natural world
because physics is nature. it explains the laws of nature. some laws already discovered. some laws yet to be discovered.
physics is the study of non living things if we added it with biology it becomes biophysics and bio is the study of living things tell me please what is this?
physics is the study of matter,energy and their interactions
all living things are matter
why rolling friction is less than sliding friction
thanks buvanas
is this a physics forum
Physics Reply
explain l-s coupling
Depk Reply
how can we say dirac equation is also called a relativistic equation in one word
preeti Reply
what is the electronic configration of Al
usman Reply
what's the signeficance of dirac equetion.?
Sibghat Reply
what is the effect of heat on refractive index
Nepal Reply
As refractive index depend on other factors also but if we supply heat on any system or media its refractive index decrease. i.e. it is inversely proportional to the heat.
you are correct
law of multiple
if we heated the ice then the refractive index be change from natural water
kia chand pr b din raat hotay hai
Practice Key Terms 4

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Source:  OpenStax, University physics volume 3. OpenStax CNX. Nov 04, 2016 Download for free at http://cnx.org/content/col12067/1.4
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