# Measuring the size of the observable universe

• g.lemaitre
In summary, the conversation discusses the size of the observable universe, which is estimated to be 93 trillion light years by some, while others argue it may be infinite. The idea that the universe is finite is based on the assumption that the size at the first Planck time was finite and using Hubble's constant and other evidence to calculate it. Some physicists believe that after inflation, the universe was only a centimeter across, which supports the idea of a finite universe. However, current experiments and models cannot distinguish between a large but finite universe and an infinite one.
g.lemaitre
The size of the observable universe usually gets put at 93 trillion light years, though some people believe it is infinite, though I'm not sure which camp is in the majority. Those who believe it is 93 trillion light years across are they just assuming that the size of the universe at the first Planck time was finite, I'm guessing 10^-35 m? And they are just using Hubble's constant, along the evidence of the extent to which Hubble's constant has sped up and slowed down over time, to calculate the size of the universe? I've read quite a few times that some physicists believe the universe was a centimeter across after inflation, hence finite. Again, let me state my question more succinctly, is the idea that the universe is finite just an assumption or is there some equation which requires it?

The size of the observable universe usually gets put at 93 trillion light years, though some people believe it is infinite
I would expect this "infinite" refers to the total universe, not just the observable part. Otherwise, can you provide a source for this claim?
The same is true for the size of the observable universe (which is not the size of the total universe) after inflation.

The universe can be finite or infinite - if it is small enough, it can be possible to measure its size, but we cannot distinguish between "really large" and "infinite" with current experiments and models.

## 1. How is the size of the observable universe measured?

The size of the observable universe is measured using various techniques, including parallax measurements, redshift measurements, and the cosmic microwave background radiation.

## 2. What is the current estimated size of the observable universe?

The current estimated size of the observable universe is approximately 93 billion light years in diameter.

## 3. Can we ever know the exact size of the observable universe?

It is unlikely that we will ever know the exact size of the observable universe due to the expansion of space and the limitations of our technology.

## 4. How does the size of the observable universe compare to the entire universe?

The observable universe is believed to be just a small fraction of the entire universe, as there may be regions of the universe that are beyond our observable reach.

## 5. Has the size of the observable universe changed over time?

Yes, the size of the observable universe has changed over time due to the expansion of space. The universe is believed to have been much smaller in the past, and will continue to expand in the future.

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