Student Ideas and Questions
Common Student Ideas
Students may think that the redshift of the light we receive from galaxies means that the galaxies themselves have turned red.
Bridge to learning:
A student that has this misconception is not taking into account that the observer sees light that is redshifted due to the motion between observer and the galaxy. If you were to view the galaxy at rest (no relative motion) you would see that the galaxy is actually giving off light that is "unshifted" and has not changed color.
Students may not understand how distances to galaxies in the Universe (measured in light-years) can exceed the actual age of the Universe (in years).
Bridge to learning:
In order to make sense of this idea, first clarify that light-years is a measure of distance, not time. Then ask students to imagine a static Universe that is not expanding. If a galaxy's light in that Universe has traveled for 1 billion years, then it would be 1 billion light years away. But what about if the Universe was expanding over that 1 billion years? Then the actual distance to the galaxy would be more than 1 billion light years away when its light reaches Earth.
Common Student Questions
If this is where the galaxy is today, where will it be tomorrow? What about in 100 years? Where was the galaxy when it emitted its light?
Due to the vast distances in space, the galaxies will appear to be at the same measured distances, whether it’s tomorrow or 100 years from now. If a galaxy is a million light years away, in 100 years there will not be a noticeable change.
Although astronomers do have ways to determine the original position of a galaxy, most are focused on the overall, average properties of the Universe - its critical density, the value of the Hubble constant, and how both of these things are changing. They have developed cosmological calculators with variables that simulate these observed conditions in the Universe. Here’s a sample cosmological calculator.
If we can already see all the way back to the edge of the observable Universe, then why do we need to build bigger telescopes? How is Rubin Observatory special, if it is not going to see farther than other telescopes?
Think of this as filling in the blanks on an incomplete picture. Current telescopes only see the biggest and brightest objects, but Rubin Observatory will see billions of faint objects. By studying the nature of these objects and how their distribution contributes to the large-scale structure of the Universe, astronomers will have better insights into how galaxies evolved and merged, and the role of dark matter and dark energy in the evolution of the Universe.
If we wait longer, will we see the light from more galaxies? If we can see all the way to the cosmic microwave background now, will these new galaxies appear before or after the cosmic microwave background?
Yes, there are new galaxies that will appear in the future, but their light will not be traveling from distances greater than our observable Universe. The light from new galaxies that appears in the future will be from galaxies that are still forming today, just as there are other objects in the Universe that have more recently begun to emit observable light, such as supernovae.
There are no galaxies beyond the distance of the cosmic microwave background, since the energy from that event happened at a time before the galaxies formed. All galaxies in the Universe are closer to Earth than the cosmic microwave background.
How big is the Universe in total? Is it infinite? How many galaxies are in it?
No one knows for certain the size of the Universe, since we can only observe galaxies at distances limited by the travel time of light (our observable Universe). However, astronomers have made estimates of the mass and density of the Universe, and these measurements suggest a Universe that is flat. A flat Universe predicts that the Universe is infinite in size.
Current estimates indicate that the radius of the Universe in total is at least 250 times the radius of the observable Universe, meaning it is at least 15 million times the observed volume. The estimates also predict there are at least 15 million times more galaxies than the estimated two trillion galaxies in our observable Universe.
Some astronomers think that our Universe may very well be infinite in size, with an infinite number of galaxies in it. Astronomers have different theories about the size of the Universe based on different calculations, but one thing they agree on is that it's either infinite, or really, really big.