World Science Scholars
2.2 A Brief History of the Universe
summary
Astronomers see a strange radiation at the furthest visible regions of the universe.

• To understand what this is, we first need to understand our place in time in the universe.
• We can learn a lot about the universe by using the sky as a “time machine.”
• When we look at distant objects, we see them as they were many years ago, since their light may have taken hundreds, millions, or even billions of years to reach us.
• Nearby galaxies appear fairly contemporary in age, but the further back we go, the “younger” the galaxies look, and beyond that, we see no galaxies at all.
• We are looking so far back in time that we are witnessing the epoch before any galaxy formations. We see merely the building blocks, mainly Hydrogen gas.

Edwin Hubble calculated the age of the universe using the distances and velocities of galaxies.

• He measured their light to be red-shifted, implying that all galaxies were moving away from us at different velocities. The further they were, the faster they were receding.
• Hubble defined the relationship between a galaxy’s distance from us and its velocity by $$v=H_0d$$ where $v$ is the velocity, $d$ is the distance, and $H_0$ is known as the Hubble constant.
• To solve for the age of the universe, $t$, $$t=\frac{d}{v} = \frac{d}{H_0d} = \frac{1}{H_0} = constant$$ We get the same answer for every galaxy, regardless of how far away it is.
• When we factor in acceleration, the value of $H_0$ ≈ 2.3 × 10-18 s-1. The age of the universe is then approximately 4.35 × 1017 seconds, or 13.8 billion years old.

Understanding the expansion of the universe helps explain this mysterious radiation at its edge.

• Since everything is expanding, so too is the gas that fills space.
• An expanding gas will cool down, so if we go backwards in time to the early stages of the universe, the gas is getting hotter and more compressed—eventually becoming a plasma.
• Therefore, what we are seeing at the edge of the universe is a plasma “screen,” an opaque wall of Hydrogen plasma.
• This is essentially a “baby picture” of our universe when it was just 400,000 years old.
• The difference in uniformity of this plasma is only about 0.001%—about as uniform as the air in a room.

Everything we see in the universe today resulted from the slight non-uniformity in the early universe.

• Small variations in mass density led to the clumping of matter.
• Bits of matter began to attract each other and formed clusters that could in turn pull in more matter with the increased gravitational force.
• Slowly, structure appeared in the universe in the form of stars, planets, galaxies, and clusters of galaxies.

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