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Learning objectives

By the end of this section, you will be able to:

  • Explain why we can observe the afterglow of the hot, early universe
  • Discuss the properties of this afterglow as we see it today, including its average temperature and the size of its temperature fluctuations
  • Describe open, flat, and curved universes and explain which type of universe is supported by observations
  • Summarize our current knowledge of the basic properties of the universe including its age and contents

The description of the first few minutes of the universe is based on theoretical calculations. It is crucial, however, that a scientific theory should be testable. What predictions does it make? And do observations show those predictions to be accurate? One success of the theory of the first few minutes of the universe is the correct prediction of the amount of helium in the universe.

Another prediction is that a significant milestone in the history of the universe occurred about 380,000 years after the Big Bang. Scientists have directly observed what the universe was like at this early stage, and these observations offer some of the strongest support for the Big Bang theory. To find out what this milestone was, let’s look at what theory tells us about what happened during the first few hundred thousand years after the Big Bang.

The fusion of helium and lithium was completed when the universe was about 4 minutes old. The universe then continued to resemble the interior of a star in some ways for a few hundred thousand years more. It remained hot and opaque, with radiation being scattered from one particle to another. It was still too hot for electrons to “settle down” and become associated with a particular nucleus; such free electrons are especially effective at scattering photons, thus ensuring that no radiation ever got very far in the early universe without having its path changed. In a way, the universe was like an enormous crowd right after a popular concert; if you get separated from a friend, even if he is wearing a flashing button, it is impossible to see through the dense crowd to spot him. Only after the crowd clears is there a path for the light from his button to reach you.

The universe becomes transparent

Not until a few hundred thousand years after the Big Bang, when the temperature had dropped to about 3000 K and the density of atomic nuclei to about 1000 per cubic centimeter, did the electrons and nuclei manage to combine to form stable atoms of hydrogen and helium ( [link] ). With no free electrons to scatter photons, the universe became transparent for the first time in cosmic history. From this point on, matter and radiation interacted much less frequently; we say that they decoupled from each other and evolved separately. Suddenly, electromagnetic radiation could really travel, and it has been traveling through the universe ever since.

Discovery of the cosmic background radiation

If the model of the universe described in the previous section is correct, then—as we look far outward in the universe and thus far back in time—the first “afterglow” of the hot, early universe should still be detectable. Observations of it would be very strong evidence that our theoretical calculations about how the universe evolved are correct. As we shall see, we have indeed detected the radiation emitted at this photon decoupling time    , when radiation began to stream freely through the universe without interacting with matter ( [link] ).

Questions & Answers

which planet orbits the closest?
Alastair Reply
What is the angle between Earth's equator and the Celestial equator? In the drawing they seem pretty similar. Thank you for this study resource.
Chuck Reply
Describe the spectrum of each of the following: starlight reflected by dust, a star behind invisible interstellar gas, and an emission nebula
shakila Reply
If the Oort cloud contains 1012 comets, and ten new comets are discovered coming close to the Sun each year, what percentage of the comets have been “used up” since the beginning of the solar system?
Day Reply
what is spectral type of sun
Akshat Reply
what everyone asking here? and who answers for them?
Shashi Reply
highest frequency wavelengh
Kathy Reply
may I know which Kingdom shows largest diversity
Arpita Reply
or students should post tough likely questions
Adepitan Reply
for example questions on demand functions and etc
Adepitan Reply
are there no ways we can get tough questions to answer
Adepitan Reply
What do you mean?
Amman Reply
excellent book bro.... keep it up.... if I find any query i will ask u.... thanks
rao Reply
Hi! I'm a bit confused, what is this
Sizakele Reply
They hsve new set of questions every time they test us. i do revision with the tutorials they give us answer extra questions from their moodle site but every time i write exams there will be few not even 10% of the questions are they. most of the time i guess. They give us three lectures who do not
Phumza Reply
Practice Key Terms 3

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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