To find this answer, multiply the age of the universe in years (14 billion years, or [image] years) by the number of seconds in one year (about [image]). Note that the far right point along the red line on the graph represents this present age of the universe.
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One billion years ago, the universe was about 13 billion years old, which is about 13/14 of its current age. On the powers of 10 scale used on the horizontal axis, this is almost indistinguishable from the present age of [image] (see Part B). Note that this present age is about 400 times the age represented by the tick mark at [image], demonstrating that 13/14 of the present age must be very close to the far right along the axis.
A point 1/14 of the way along the horizontal axis represents an age of about [image], which is a very tiny fraction of a second.
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almost all the way to the far right
One billion years ago, the universe was about 13 billion years old, which is about 13/14 of its current age. On the powers of 10 scale used on the horizontal axis, this is almost indistinguishable from the present age of [image] (see Part B). Note that this present age is about 400 times the age represented by the tick mark at [image], demonstrating that 13/14 of the present age must be very close to the far right along the axis.
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Recall that, in scientific notation, [image] Along the horizontal axis, this point is the tick mark halfway between the tick marks labeled [image] and [image]. From the height of the red line, this point corresponds to a temperature of about [image].
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about 3,000,000,000 times
The temperature of the universe at an age of 1 second was about [image] , or 10,000,000,000 [image] (see Part D), which is about 3,000,000,000 times the current temperature of about 3 [image].
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fell very rapidly when the universe was young, but is changing very gradually today
This fact leads to a very important point about any graph that is labeled with powers of 10 on either or both axes: When the graph uses a power-of-10 scale, a straight line does not have the same linear interpretation as it does on a graph that uses a linear scale on both axes.
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Be sure you understand the changes that marked the transitions between eras, as well as conditions during the eras themselves.
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That is because the universe was very hot when it was born, and has steadily cooled with time.
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he universe is expanding with time, which makes the observable universe grow larger in size as time passes. (Note that the “observable universe” means the size of the universe as it would appear to any hypothetical observer located any place within it.)
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The universe has been expanding since its birth in the Big Bang, and this expansion causes the universe as a whole to cool with time.
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The cosmic microwave background is essentially the thermal radiation emitted by the universe as a whole, and the laws of thermal radiation tell us that hotter objects emit light with a shorter peak wavelength (higher energy). Because the universe cools with time (see Part A), we conclude that the peak wavelength of the cosmic microwave background must get longer as time passes.
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Individual photons stretch to longer (redder) wavelength as the universe expands; this is the phenomenon sometimes referred to as cosmological redshift.
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Most matter in the early universe was annihilated by antimatter.
Although most matter was annihilated, virtually all antimatter suffered this fate. That is why antimatter has been very rare in the universe ever since that time, which marked the end of what we call the "particle era."
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Spacetime rapidly expanded during a brief period of inflation.
As you can see in the interactive figure, inflation is thought to have occurred when the universe was only a tiny fraction of a second old.
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Light began to travel freely through the universe.
This time when light could first travel freely marks the release of the cosmic microwave background, which we can still observe today.
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by the time the universe was about 3 minutes old
Hydrogen nuclei are protons, and protons were formed during the particle era. During the next three minutes, many of these protons fused and then broke back apart, but by the end of that period these reactions stopped. Essentially all the hydrogen that will ever exist was therefore present in the universe when the universe was just 3 minutes old, because subsequent fusion in stars has consumed some of this hydrogen but has not made any more.
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fainter and has most of its photons at longer wavelengths
The universe is expanding and cooling with time, causing the cosmic microwave photons to stretch to longer wavelengths.
