The Size and Shape of the Universe

The Uniform Temperature of the Early Universe The brighter and darker spots in the microwave background correspond to hotter and cooler areas in the universe (at an age of 400,000 years). However, the hotter spots are only a tiny bit hotter than the cooler spots. So all parts of the observable universe were nearly the same temperature at that time.

If all parts of the universe had nearly the same temperature, that implies that heat had flowed from hotter areas to colder areas to even out the temperature. However, at that point in time (age = 400,000 years), the observable universe had a diameter of roughly 100 million light years, and heat cannot flow faster than the speed of light. heat The Uniform Temperature of the Early Universe

Inflation How could parts of the universe separated by millions of light years have the same temperature when the universe had an age of only 400,000 years? Here’s the best explanation: Immediately after the Big Bang, the universe was small enough for heat to flow between different parts of the universe, producing a uniform temperature. But the universe then expanded extremely rapidly, much faster than the speed of light. This period of rapid expansion is called inflation.

Inflation Nothing in the universe can move faster than light, but the universe itself is not restricted to this speed limit as it expands. During inflation, which lasted for only a tiny fraction of a second, the universe grew from the size of an atom to 1 billion light years across!

The Size of the Universe The observable universe consists of the portion of the universe that is close enough so that light from it has had enough time to reach us since the Universe was born. This region is a sphere centered on the Earth, and its boundary is called the light horizon.

The Size of the Universe The microwave background radiation that we see today was produced shortly after the Big Bang by material that was only 36 million light years from our position in the universe, and yet it took 13 billion years to arrive at our location because of the expansion of the universe. after Big Bang: The matter that produced the microwave photons that we see today is now 46 billion light years away, probably in the form of a galaxy. So the current diameter of the observable universe is 92 billion light years. Now: our location 46 billion light years 42 million light years CMB seen today

Beyond the Observable Universe Because the universe has expanded faster than the speed of light, some areas of the universe have been pulled beyond our light horizon, and are outside of our observable universe.

Beyond the Observable Universe If we wait long enough, light will eventually reach us from some of the areas currently outside of our observable universe. But other areas are too far away for their light to ever catch up and reach us as the universe expands. Those areas will always remain outside of our observable universe. Theories of inflation suggest that the true size of the universe may be times larger than our observable universe, or light years. And it’s still expanding, so it will grow even larger!

The density of matter and energy (  ) in the universe determines the shape of the universe since they curve and distort space. There are 3 possible shapes for a 3-D universe. We can’t visualize them, but we can draw 2-D versions of these shapes. The Shape of the Universe  >1: space curves so much that it wraps back on itself; universe has positive curvature and is finite; this is a closed universe  <1: the universe has negative curvature and is infinite; this is an open universe  =1: the universe has no curvature and is infinite; this is a flat universe

The Shape of the Universe As light travels through the universe, its path will follow the curvature of space. As a result, the apparent size of an object from our point of view is affected by the shape of the universe. We can determine this shape by measuring the sizes of clumps in the microwave background. By doing so, we find that the universe is flat.

There are an infinite number of possible shapes for closed and open universes (i.e., infinite number of values of  1), but only one flat shape (  =1). So why do we happen to live in a flat universe? The theory of inflation is able to explain why: Because of inflation, the universe has expanded far beyond our observable universe. As a result, regardless of the true shape of the universe, it will appear nearly flat within our much smaller observable universe. We’ll never be able to measure the true shape of the universe as a whole. Why is the universe flat? entire universe observable universe inflation