(THE) MODERN
ASTRONOMICAL VIEW OF THE WORLD
ASTRONOMICAL VIEW OF THE WORLD – LECTURE 10
JONI TAMMI
NEXT WEEKS
L11: Art and Science
fiction
L12: Astrobiology and
life in the Universe
L 13: Big open questions
Smaller topics:
Preliminary work in
MyCourses
Esko Valtaoja, prof. emeritus
• Light pollution
• Galaxies
• Something else?
MyCourses: Excursion 16.3.2017
18:00-20:00
Registration open now, until next lecture.
Closing the course
TODAY
1. Cosmology & the Universe
2. Relativity & “space-time”
PRELIMINARY WORK
Quote of the week:
• Space-time
• World-lines
• Speed of light (in vacuum) is constant, 1 c
• Gravity ~ curvature of space-time
“Matter tells space how to curve.
Space tells matter how to move.”
John Archibald Wheeler
COSMOLOGY
PART 1
STORY SO FAR (LECTURE 8)
Expansion of the Universe
• Galaxies are moving away from each others
Hubble time, Universe’s lifetime
• The distance between galaxies is growing
• Hubble’s law: v = H0 D
H0 = 68 km/s/Mpc
• Universe is expanding
t = 13.8 Gy
• Universe has a finite lifetime
Hubble constant tells us
how fast the Universe
expands, and how old it is.
So what happened 13.8 milliard/billion years ago?
E.g. Wikipedia: Long vs short scale
COMMON MISCONCEPTIONS ABOUT THE BIG BANG
•
•
•
•
•
•
•
•
It was a big bang
It was an explosion
All current matter was in one place
Galaxies are flying away from the
explosion
It happened at some specific place
It happened in the centre of the
universe
Because everything is getting away
from us, we are in the centre of the
universe
”What happened before BB?”
Origin of the name:
A steady-state-universe proponent
ridiculing ”this ’big bang’ idea”.
Finnish: ”alkuräjähdys”, initial explosion.
WHAT CAUSED THE BIG BANG?
Our universe began (or restarted?) in the BB. But
did something happen outside our universe that
caused the BB?
Some guesses:
Many ideas exist, but nothing concrete yet. Field
of pre-big-bang cosmologist, not astronomers.
Very little data available, few theories can be
ruled out.
Beware: ”god of the gaps” lives here, one of the
last ”battlegrounds” between science-vs-religion
enthusiasts.
• Quantum-fluctuations creating
the initial singularity?
• Collision of two branes
(Ekpyrotic universe)?
• Followed the Big Crunch of the
previous universe (cyclic universe)?
How did we get here from the BB?
TL; DR:
COSMOLOGY
IN ONE
PICTURE:
BIG BANG TIMELINE
Step 0: Big Bang
About 13.8 × 109 years ago
• Time begins
• Size of the universe = 0
• Energy density = infinite
• No matter
Hey, Joni in the future! Remember the
“story of cooling and reaching certain thresholds”.
Draw the coordinates already!
And tell the students not to worry too much about
the particle-level stuff, but to focus on the big picture.
- Joni from the past
Ps. You look great, is that a new shirt?
• The universe starts to expand
• Volume increases  density drops, cools down
BIG BANG TIMELINE
Step 1: Inflation
~10-32 seconds after BB
Step 0: Big Bang
• After it’s birth, the universe expands
exponentially for a short time.
• Growth of the order of at least 1026
• Space grows faster than the speed of light.
(OK, because expansion ≠ movement)
• After inflation is over, BB expansion begins.
• From this point on, physics pretty much
understood.
BIG BANG TIMELINE
Step 2: Hadrons form
~1 second after BB
Step 1: Inflation
• Temperature has dropped low enough (1010 K)
so that hadrons (e.g. protons & neutrons) have
started to form.
Step 0: Big Bang
• Both matter and antimatter, but a tiny bit more
matter. For each 108 antiprotons we get one
extra proton.
• Most get annihilated (matter and antimatter),
leaving only little matter and a lot of photons.
BIG BANG TIMELINE
Step 3: Leptons dominate
~10 second after BB
Step 2: Hardons form
• Most hadrons/anti-hadrons have annihilated;
mostly leptons (e.g. electrons) left.
Step 1: Inflation
Step 0: Big Bang
• Most leptons and anti-leptons annihilate,
leaving mostly photons.
• Universe mostly filled with photons.
BIG BANG TIMELINE
Step 4: Nucleosynthesis
3 –10 minutes after BB
Step 3: Leptons dominate
• Temperature has dropped to 109 K.
Step 2: Hardons form
• Protons and neutrons start to form nuclei.
Step 1: Inflation
• 75 % Hydrogen, 25 % Helium (by mass).
Step 0: Big Bang
• Nuclei scatter photons around.
BIG BANG TIMELINE
Step 5: Recombination & decoupling 380 000 years after BB
Step 4: Nucleosynthesis
• Temperature dropped to 3000 K.
Step 3: Leptons dominate
• Protons and electrons get together
(recombine) to make neutral hydrogen.
Step 2: Hardons form
Step 1: Inflation
Step 0: Big Bang
• Result: Decoupling.
Potons are no longer constantly thrown
around by nuclei, but can travel freely. Light
decouples from matter.
• Photons scatter one last time off the
380 000-year-old plasma, and fly away.
COSMIC MICROWAVE BACKGROUND
Here are the
photons now.
This is a
picture of
the universe
at the moment when
light and matter decoupled,
380 000 years after the big bang.
Details in a few minutes
BIG BANG TIMELINE
Step 6: Stars and galaxies
100–400 million year after BB
Step 5: Recombination & decoupling
• First stars and galaxies form.
Step 4: Nucleosynthesis
• First supernovas (heavier elements).
Step 3: Leptons dominate
Step 2: Hardons form
Step 1: Inflation
Step 0: Big Bang
BIG BANG TIMELINE
Step 7: Acceleration kicks in
~5 billion years after BB
Step 6: Stars and galaxies
• Energy density of matter and radiation
keep dropping.
Step 5: Recombination & decoupling
Step 4: Nucleosynthesis
Step 3: Leptons dominate
Step 2: Hardons form
• At this point, their effect to the behaviour of
the Universe becomes weaker than another
component, which starts to dominate.
Step 1: Inflation
• As a result, Universe’s expansion starts to
accelerate.
Step 0: Big Bang
• This component is called the Dark Energy.
Each line = one possible ”life” of
the Universe.
Which one our Universe follows can
be determined from observations.
BIG BANG TIMELINE
Step 7: Dark energy steps in
~5 billion years after BB
Step 6: Stars and galaxies
• Energy density of matter and radiation
keep dropping.
Step 5: Recombination & decoupling
Step 4: Nucleosynthesis
Step 3: Leptons dominate
Step 2: Hardons form
• At this point, their effect to the behaviour of
the Universe becomes weaker than another
component, which starts to dominate.
Step 1: Inflation
• As a result, Universe’s expansion starts to
accelerate.
Step 0: Big Bang
• This component is called the Dark Energy.
(Details beyond this course, for more information start from
Cosmological constant and Dark energy and follow the references.)
BIG BANG TIMELINE
Step 8: Now
13.8 billion years after BB
Step 7: Dark energy steps in
Step 6: Stars and galaxies
• The photons from the ”last scattering” have
redshifted to microwave energies.
Step 5: Recombination & decoupling
• Temperature 2.725 K
Step 4: Nucleosynthesis
• Expansion keeps accelerating, temperature
keeps decreasing.
Step 3: Leptons dominate
Step 2: Hardons form
Step 1: Inflation
Step 0: Big Bang
• (Energy) Content of the Universe is a
mixture of matter and energy:
CONTENT OF THE UNIVERSE
Dark
Matter
27%
Normal
Matter
5%
Dark
Energy
68%
DARK MATTER
Matter in/around galaxies
that doesn’t emit light, but
whose gravity can be seen.
Composition not yet known.
Probably some new particle.
Detection, for example, by
Kepler’s laws (galaxy’s rotation)
General relativity
(gravitational lense)
DARK ENERGY
”Hypothetical form of energy”
that causes the expansion of the
universe to accelerate.
Integration constant  in Einstein’s
equations.
E.g. constant energy of space,
vacuum energy, etc.
CONTENT OF THE UNIVERSE
Dark
Matter
27%
Normal
Matter
5%
Dark
Energy
68%
NORMAL MATTER (CLOSER LOOK)
Neutrinos
0,3 %
Heavy
elements
0,03 %
Stars
0,5 %
Free gas
4%
CONTENT OF THE UNIVERSE
Visible
things
0.53%
Dark
Matter
27%
Other
normal matter
4,5 %
Dark
Energy
68%
SUMMARY
70 % of the
universe is
something we
don’t understand,
but at least we
know it’s not
matter.
85 % of all
matter is
something we
don’t understand,
but at least we
can see it’s there.
RELATIVITY AND SPACETIME
PART 2
RELATIVITY: SPECIAL (NARROW) AND GENERAL
Special relativity (1905)
General relativity (1916)
• Things moving fast, but at constant speed • Generalised theory (also much more complicated)
• Straight and no acceleration
• Acceleration
• In free space (no gravity)
• Gravity
• Galilei & Newton: things look the same
regardless of being at rest or moving
smoothly. Both are equal and relative.
Only time is absolute and same for all.
• Einstein: also time is relative.
• Only the speed of light is the same for
everyone(!).
• 4-dimensional spacetime: x, y, z, t
• Things look the same regardless of being in a
gravity field or accelerating.
• Gravity: property of spacetime rather than a
traditional interaction/force.
• Mass ”bends” the spacetime
• Things move straight lines, it’s the space that curves
RELATIVITY: SPECIAL (NARROW) AND GENERAL
Special relativity (1905)
General relativity (1916)
• Relativistic speeds
• Black holes
• Slowing time
• Einstein’s crosses and rings
• Increasing mass
• Perihelion shifts
• …
• …
NEWTON VS. EINSTEIN (SPECIAL RELATIVITY)
Newtonian physics:
Perfect for speeds below ~10%
of the speed of light (0.1c).
Energy vs. speed
10
Γ=
9
1
𝑣
1− 𝑐
2
7
6
5
4
3
Difference:
Lorentz factor Γ
E.g. 𝑚 ⟶ 𝑚 × Γ, 𝑡 ⟶
Lorentz factor Γ
8
Special relativity:
Works on all speeds.
For speeds > 0.1 c starts to
differ from Newtonian.
goes to
infinity
2
𝑡
Γ
1
0%
20%
Newton
Einstein
40%
60%
Speed (% c)
80%
100%
goes to
infinity
Energy vs. speed
10
9
Life
Lorentz factor Γ
8
Common sense ends here
Space
shuttle
Cosmic
rays
Solar
wind
7
6
Supernovas
5
4
Quasars
3
2
1
0%
20%
Newton
Einstein
40%
60%
Speed (% c)
80%
100%
goes to
infinity
Energy vs. speed
• If you add velocities together,
they will not add up to over the
speed of light.
• ”Speed of time” depends on
where you are and how fast you
are moving.
• There’s no such thing that “at the
same time”, in the traditional
meaning; the order of evends
depends on whom you ask.
10
9
Life
8
Lorentz factor Γ
Special relativity in three
sentences:
Cosmic
rays
7
6
5
4
Quasars
3
2
1
0%
20%
40%
60%
Speed (% c)
80%
100%
THE MAIN POINTS & LIMITATIONS
If something starts with 1c
that’s no problem.
• Nothing can be accelerated to the speed of light.
• Nothing can move through space faster than light.
• Time is not constant.
• Everything is relative.
But not certain if we can move spacetime
itself, or ”cheat” some other way.
See, e.g., Alcubierre drive or
NASA's plans on warp drives, or good old
Wormholes.
WARNING: THE ”PROBLEM” WITH RELATIVITY
If
we have a metal rod that is one
light-year long and infinitely rigid,
and we push it, we make the
other end move immediately,
then we have sent information faster
than the speed of light,
i.e., the laws of physics are broken.
If
we assume that the
laws of physics are broken,
and we do things that
would break the laws of physics,
then our results are consistent with
the initial hypothesis,
i.e., the laws of physics are broken.
tl;dr:
Assumption: laws of nature don’t work.
Conclusion: laws of nature don’t work.
NEXT WEEK: ART & SCIENCE FICTION
Details on MyCourses (Week 11)
Preliminary work, to be submitted on
MyCourses before Mon 13.3. 08:00:
“Share your astronomical inspiration”
10-15 presentations chosen, the rest to be read
online.
Prepare to give a 3-minute presentation on a piece
of art (however you define it):
a) what is the astronomical content of the piece,
Grading:
b) why did it touch or amaze you, and, if relevant,
Did you return your 1-3 –slide PDF
before the deadline:
c) how it has changed your worldview (if it did),
Yes:
No:
6 points
0 points
d) how does it correspond to your chosen field of
study/work (if it does).
Relativity paradoxes
[Left here just in case there is time at the end of the lecture. If not, feel free to read on, and find
more information about the “illogical” theory of relativity.
IT DOESN’T HAVE TO BE INTUITIVE IN ORDER TO BE CORRECT
If you are on a train moving at 100 km/h,
If you add velocities together,
they will not add up to over the
speed of light.
Vbullet
and shoot forward a bullet with speed 100 km/h,
you measure for its speed 100 km/h,
and observer on the ground measures 200 km/h.
If you are on a train moving at 0.9 c,
and shoot forward a bullet with speed 0.9 c,
you measure for its speed 0.9 c,
and observer on the ground measures 0.995 c.
Vtrain
IT DOESN’T HAVE TO BE INTUITIVE IN ORDER TO BE CORRECT
Year
3000: You are born.
3020: Your child is born.
”Speed of time” depends on
where you are and how fast
you are moving.
3030: You are 30, your child is 10.
You (alone) visit a star 14 ly away with
a spaceship moving at 99 % speed of light.
The trip takes 30 years.
3060: You return.
Your child is 40.
You are 35.
You were born 60 year ago. You have lived 35 years.