How to Visualize the Universe in Your Head

If you search for pictures of the universe online, here is what you get.  It's a crazy zoo of cartoons and drawings.   If you look at them carefully though, there are two popular views.  One view is from the side.  Our universe looks like some kind of trumpet or funnel, with the Big Bang on the left, growing bigger as time increases toward the right.   The other common view looks like a big ball filled with stars and galaxies.   Sometimes the universe is drawn with a line around it indicating some kind of boundary, sometimes they show it as just a ball of stars and galaxies without any obvious boundary.  Let's talk about whether any of these "pictures" are useful models or just misleading cartoons.

Some pictures of the universe [Google]

 

In particular I am interested in the following questions.

1.  Does our universe have a boundary? 

Does our universe have a boundary?    Has anyone ever detected a boundary?  If there is no boundary then is our universe infinitely large?  How should we imagine that!  If there is a boundary, then what is outside the boundary?  Nothing?  Other universes?

If there is a boundary, then what lies
outside the boundary?  [image from Wikipedia]

2. Is space expanding?

Is space expanding, like a stretching rubber sheet or a rising loaf of raisin bread?  Am I expanding? Is San Francisco expanding?  Are all the atoms and molecules in the universe expanding?

Raisin Bread [YouTube]

First the boundary question.

Below is a ball-shaped picture of the universe made by the Planck Observatory.   This map has us (the observer) at the center and the Cosmic Microwave Background (CMB) as the outside "skin."  There is certainly a boundary and it is clearly visible.   The Planck telescope is at the center of the sphere, looking in all directions.   The telescope sees nearby stars, far-away galaxies, and farthest of all the CMB radiation, which is light emitted just after the Big Bang.  

Map of the observable universe from the Planck
Observatory, showing the CMB event.

This picture above is a map of the observable universe, and it takes a minute to understand what is shown here.

Let's look at a slice of the ball (below).  "You Are Here" is at the bottom corner.  The scale on the right is billions of years and the scale on the left is redshift.   The farthest thing we can see is at 13.7 billion years lookback time.  It's the bright flash of light that came just after the big bang.   We can't see the light from the actual big bang itself because the universe was opaque to light at that time.  So the CMB is the boundary of the (observable) universe.  What is beyond that boundary? The big bang, at 13.8 billion years.  What is beyond (before) the big bang?  That's anybody's guess.  Maybe Nothing.

Bottom line: Yes, the observable universe has a distinct, visible boundary.  It was photographed by the Planck observatory.

Map of the observable universe.
[from mapoftheuniverse.net]

2. Is space expanding?

Let's look at one of the side view pictures, from NASA.   This one is shaped like a funnel.  It shows the  universe is getting bigger and bigger as time goes on.   This one has a lot of labels, but most importantly the big bang is on the left and "now" is on the right.   It's pretty obvious that there is a boundary on the left and no boundary on the right.   So time (and space) begins with the big bang and it is open-ended towards the future.

This (above) is a fancy drawing but very misleading.  It looks like the universe is a huge semi-transparent plastic cup full of stars and galaxies.    That's nonsense.   What is outside the "cup?"  Shouldn't the timelines of stars go from birth to death (several billion years)?  If the horizontal axis is time, how can they show galaxies like that?  It's not very useful.  

Below is a more useful recent plot, a "Hubble Diagram."  In 1929 Edwin Hubble  realized that all the stars and galaxies are receding from us, and the recession velocity is proportional to their distance.

Hubble Diagram

This is Hubble's Law.  I am a big fan.  It says that the recession velocity of an object is proportional to its distance.  Nobody can argue with that data.  It has been confirmed and refined over the last 100 years. But there is still debate today about the interpretation.   What does "velocity" really mean?  Is the redshift purely Doppler redshift?  Can empty space "expand?"  How does that work?  What does that even mean? 

Einstein's general relativity theory is very complex and isn't easy to visualize, but there is a trick that I use.  When space is curved (like on earth) we feel the gravitational field.  When it's not curved, there is no field.  So to visualize curved space, just think of it as a gravitational field.  More field, more curvature.  (This is actually Einstein's equivalence principle in simple terms).

  

In any case, Hubble's Law tells us that stars and galaxies are certainly moving away from us at very high speed.  It certainly does not imply that WE are getting bigger.  Our own galaxy is not "expanding."

As far as I can tell the big question about whether our universe is finite or infinite has not been answered yet.  Maybe soon.

Here is the picture I like to imagine in my head when I try to imagine Hubble's Law.  It's a firework explosion.

Big Bang

[Thanks to GIPHY]

Please end me your thoughts, comments and opinions (private or public).

Al Kordesch

Aug 31, 2023