Hey Crazies.
We just finished a 4-video series on cosmology.
If you haven’t seen those videos yet, you
should probably go watch those first.
I put them in a playlist.
I responded to a bunch of comments throughout
those videos, but I wanted to take some time
and get to some good ones that I missed.
First, I’d like to say hello to everyone
who came over from Looking Glass Universe.
Welcome to the Science Asylum. I hope you
like what you’ve seen.
Second! How’s this, Deceived Angel. Big
enough?!
Third! Let’s get into the questions.
Abu Sayed asked why the universe is the
same everywhere.
This is actually a question that took scientists
a long time to explain.
Cue Inflation Theory!
We look at images of the Cosmic Microwave
Background and see that even as early as 13.8
billion years ago, the universe was pretty
uniform.
If the universe was always expanding at the
current rate that wouldn’t have been possible.
Uniformity requires interaction between different parts of the universe.
and there wasn't enough time for that.
But if the expansion started out slow enough
to let everything interact followed it up with
an extra fast expansion, then it could make up
for lost time.
That extra fast expansion is called “Inflation.”
Jim Groth asked if the Cosmic Microwave Background
is the same distance from us in every direction.
Yes. Yes, it is,
but this isn’t really about distance. It’s
about time.
That’s a sky map of microwave light that’s
been traveling through the universe for 13.8
billion years and it’s just finally arrived
at Earth.
This is related to Caleb Lyda’s question
about diagonal lines in a spacetime diagram.
Vertical distance in this diagram is actually
time.
The slanted 45 degree lines are the spacetime
paths of light on its way to Earth.
CMB light was emitted by super-hot hydrogen
gas 13.8 billion ly away 13.8 billion years
ago.
Two equal sides of a triangle make a third
side slanted at 45 degrees.
Light travels at a speed of 1 light year per
year, so it will always be on a 45 degree
line in a spacetime diagram.
Shallower slopes require faster-than-light
travel.
Speaking of spacetime, it’s not actually
a fabric. That’s just an analogy.
It doesn’t work like a rubber sheet.
The Big Bang was not an explosion.
It’s just a time when the universe began
expanding,
for whatever reason.
The term “Big Bang” was coined by Fred
Hoyle as an insult to the idea.
It’s a very misleading name.
There is no single place in the universe that
all galaxies came from.
It’s space itself that’s expanding.
So all the places that all the observable
galaxies occupy, were once one place.
The galaxies aren’t moving apart. The space
they all occupy is getting bigger.
Speaking of which
Akash Shandilya questioned what I said about
cosmological redshift not being due to the
Doppler Effect.
Hmmm ok, we’ll do this.
This is the equation for cosmological redshift,
redshift due to expansion.
This is the equation for Doppler redshift,
redshift due to motion.
The truth is, when it comes to distant galaxies,
these are actually consistent with each other.
They’re both equations from relativity.
We should expect them to be consistent.
That doesn’t mean that Doppler is the cause
though.
What’s actually happening is the expansion
is causing a redshift and also causing a motion
in distant galaxies from our point of view.
The motion is still relative, so the Doppler
equation still works.
But it’s important to understand that the fundamental
cause is the expansion of the space.
Jo Reven thinks this lack of knowledge about
dark energy and dark matter is enough to say
they don’t exist.
I disagree.
First, absence of evidence is not evidence
of absence.
Second, we have a lot of evidence for these
things, so let’s clarify what we know and
what we don’t know.
Dark Matter is called “dark” because,
well, it’s dark.
We receive very little light from it, if any.
But we can see if it blocks things, so we
know it’s there.
I did a video a while back on dark matter
if you’re interested.
Dark Energy is called “dark” because we
don’t know what it is.
We’re in the dark about it.
We don't know what it is.
And we call it energy because this term is
an energy density.
But, just because we don’t know what it
is, it doesn’t mean it’s not there.
Dark energy is necessary to balance our equations.
Since our universe is flat, curvature is zero.
This is regular matter, dark matter, photons,
etc.
Basically, everything that isn’t dark energy
and there’s not enough of it to flatten
the universe.
Whatever dark energy is, we need it to make
up the difference.
Thanks to everyone else in that thread for
helping me answer the tough questions.
Next, the elephant in the room.
Not an actual
elephant!
A couple people pointed out that if the CMB
is redshifting , then it’s losing energy
all this time.
That’s actually true.
Physics Girl did a video on this.
What will blow your mind even more is that
dark energy density has to stay constant.
It’s the Cosmological Constant.
That means, as the universe grows in size,
there’s more and more dark energy
and that dark energy is increasing faster
than the CMB energy is decreasing.
We actually have an energy creation problem,
not an energy loss problem.
PBS Space Time did a video on this with more
detail.
Alright, some last minute quickie things!
If you’re into general conversations about
science, Rick Clark left a great comment about
non-scientific ideas masquerading as science.
It could use some love and attention.
Next, the end-of-the-universe tally came down
like this.
We had 3 votes for the big bounce, 2 for the
big crunch, 1 for the big freeze and zero
for the big rip.
Yeah, I also have a hard time believing that last
possibility too.
Arby B C noticed the subtle plug for my eBook
so I’ll be a little less subtle now.
I have an advanced physics eBook available
for PDF download.
I self-published it in June 2015 to avoid
publisher politics.
It took me 6 years; I loved writing it & feel
pretty great about what it’s become.
In June 2016, I released an update with a
clickable table of contents, index, etc.
Now it’s June 2017 and I’m releasing another
update.
I was inspired by this video series, so I
added a little more cosmology stuff.
If you’ve already downloaded it, you should
receive a link by email where you can get
the updated copy for free.
Ok crazies, keep those comments coming
and until next time, remember, it’s okay
to be a little crazy.
