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  • Writer's pictureLuke

Bang Crunch Bounce Rip

Updated: Mar 12, 2021

I still think it’s funny that they say the universe was conceived in a “big bang”.... I know it’s just as childish as laughing at “Uranus” but come on! “Big Bang”! Really!? This is the start of the universe, sound wouldn’t exist and the physicists that coined the term had to know that! I think ‘bang’ never meant sound... It was an inside joke that went too far and now no one wants to fess up to correct it, it just doesn't make sense any other way. Part of why it’s too late to change this inside joke is this terminology has been exported to some of the theories of the end of the universe as well such as; the “big crunch” "big bounce" and the “big rip”.

rtist's conception of the observable universe with the Solar System at the center,with a logarithmic scale showing the universe further out: Kuiper belt, Oort cloud, Alpha Centauri, Perseus Arm, Milky Way galaxy, Andromeda galaxy, nearby galaxies, Cosmic Web of galaxies, Cosmic microwave radiation and the Big Bang's invisible plasma on the edge.

Let's start just like everything else with the big bang. When looking at distant galaxies we have noticed that the vast majority of them are traveling away from us. And the ones that are further from us are going away from us faster than ones that are nearby. We make sense of this observation by saying the universe is expanding.

As the universe expands things that are farther away move farther away faster. In this picture The raisins represent galaxies. One raisin is 5cm the other 10cm when baked the bread becomes twice as big moving the near raisin 5cm (to 10cm away) while moving the far raisin 10 cm (to 20cm away) over the same amount of time.

Like baking a raisin bread when you put dough in an oven and take it out later and it has doubled in size as bread. If you saw this in the perspective of a raisin you might start in the dough with raisins spaced uniformly 1cm apart meaning a raisin's closest neighbor raisin would be 1 cm away next is 2 cm next 3cm and so on. Once the bread doubled in size the closest one would now be 2cm away next 4cm, 6cm and so on. This doubling effect would appear to the raisin that everything was moving away from it with things further away from it moving faster away.

Using this observation we can infer that if we rewind the clock we would find that earlier in the universe these galaxies were much closer to us. And rewinding all the way back it’s thought that there was once a time that everything in the universe was tightly packed in one place and started t=expanding from there we call this “the big Bang” *snickers*

Timeline of the universe. Starting with the big bang at the far left and ending 13.77 billion years later with the present day at far right.

Now let’s turn the clock forward and see what this could predict for the future. As the universe currently expands there’s 3 ways this could end:

  1. The universe stops expanding and starts contracting

  2. The universe stops expanding at some point and stays static

  3. The universe expands forever

The first scenario seems to be favored by most people I talk to, I think it's human nature to favor cyclical or symmetric cycles of the universe forming at one point, expanding and then boomerang back to contracting to one point. This is possible as the force of gravity pulls mass together so it would be conceivable at some point this force will slow the expansion to a stop and revise to pull everything back together. This is called “the big bounce” or “the big crunch”. If the universe contracts back to one point and ends there it would be a big crunch. If once the universe is back to one point it starts to recycle and gives rise to a new universe, in a new big bang, then it would be a big bounce.

Galaxies merging in a “big crunch” this might result in a new universe but life in this universe wouldn’t be able to survive

The second scenario where the universe just stops expanding and stays still seems to be the least likely and most boring. This is often called a “flat universe” where the universe will find some equilibrium and remain static until heat death (see previous blog) or some other fate ends the universe.

The last scenario of the universe continually expanding seems to currently have the most evidence to support it. There is no indication that the expansion is slowing at all, actually it appears to be speeding up if anything. There seems to be an unknown force that is pushing to expand faster and faster. We call this force Dark energy, If you think back to the raisin bread analogy, dark energy is like the yeast that makes the bubbles that ultimately makes the bread expand. This is called “the big rip” as there will be a point where all galaxies that are not tied to our gravity will be traveling so fast so far away from us that not even their light will reach us. Future astronomers might not believe or have the records we keep of other galaxies and become convinced that the universe is only our galaxy or our local group of galaxies. It’s kind of ironic that the scenario of an ever expanding universe ends with life having access to a smaller and smaller portion of the universe.

a graph showing the size of the universe over time omega is basically a ratio between the force of dark energy over the force of gravity if the forces equal out ohm=1 and it is a flat universe if gravity is bigger ohm>1 and we have a big crunch/bounce if dark energy is greater than gravity then ohm<1 and universe will expand forever.

So it all comes down to a fight between the known Gravity and the unknown Dark Energy. If Gravity is stronger than Dark Energy then we will have a big crunch or big bounce. If gravity is exactly equal in strength to dark energy we will have a flat universe. And finally if dark energy is stronger than gravity our universe will end in the big rip.

So could life survive any of these ends?... probably not but maybe. Famously in the hard SciFi novel “Tau Zero” author Poul Anderson describes how a crew on a spaceship with a damaged engine is propelled outside the condensing universe and watches as a big bounce happens the universe condenses until they watch another big bang create a new universe.

Tau Zero a hard science fiction novel by American writer Poul Anderson. The name comes from time dilation tau being proper time and as the ship gets closer to the speed of light tau approaches zero. A small tau means big time dilation towards the end tau is so small the crew experiences "billion-year cycles which pass as moments"

Outside of Tau Zero, if the universe is cyclic like this it’s probable that this might not have been the first cycle of the universe and our very presence shows that life can survive a big crunch or at least sometimes develops/redevelopes between bangs and crunches.

Surviving a flat universe is simple as there is no cataclysmic end that is brought on by that scenario, life could explore the static universe until some other fate puts an end to it.

The big rip on the other hand might be the hardest one to survive as so little is known about dark energy. The big rip wouldn't necessarily harm life directly. Dark energy dose not seem to have a measurable effect on much immediately around up (our solar system, galaxy, or local group of galaxies). However, because so little is known about dark energy this could change and cause us much more immediate problems problems

But even a big rip effecting only the outer galaxies would limit resources available to life. Making it harder to survive something else like heat death. We would need some serious SciFi equipment to continue exploring the universe in a big rip like, worm holes, or have life immediately expand to the outer reaches of our universe so that once it is expanded to be too far to get it would be already seeded with life. Optimistically maybe once more is discovered about dark energy it could be used or harness somehow to create the solutions we would need like faster than light travel to combat a big rip.

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