Big Questions of the Universe Flashcards
The Big Bang
observation: the expanding universe
hypothesis: the big bang
- The Universe must have been extremely small, compact
- The Universe must have begun in a big bang
The Big Bang: Observation
the expanding universe
The Big Bang: Hypothesis
the big bang
- The Universe must have been extremely small, compact
- The Universe must have begun in a big bang
The Big Bang: Prediction
Using our existing understanding of physics, we would expect that if Universe were once contained in a single point and expanded outwards, then…
- The Early Universe would be smooth – small density fluctuations
- The Large-scale structures of the Universe should be homogenous
- There should be hot residual radiation from the early Universe
The Big Bang: Experiment
Bell Lab’s horn antenna stumbled upon cosmic microwave background radiation
The Big Bang: Observation II
The Cosmic Microwave Background
- Thermal radiation dating to 380 000 years after the big bang
- Minute fluctuations in density correspond to slight differences in temperature
Observation agrees with predictions
The Big Bang: Observation II
The Cosmic Microwave Background
- Thermal radiation dating to 380 000 years after the big bang
- Minute fluctuations in density correspond to slight differences in temperature
Observation agrees with predictions
Causes of the Big Bang
What drove this explosive growth?
Was there anything before the Big Bang?
What is the nature of space and time on
Can we observe the Big Bang?
- Can’t see beyond the opaque CMB
- Gravitational waves can permeate the CMB
The Invisible Universe
We can only directly observe 5% of the Universe!
Only 5% of the mass-energy content of the Universe consists of baryonic matter (ordinary
matter)
What is the nature of Dark Matter?
What is the nature of Dark Energy?
Dark Matter
We can observe the (gravitational effects) of Dark Matter
It can be ‘seen’ in rotation curves and gravitation lensing
- All galaxies are immersed in halos of dark matter
Dark matter is thought to have provided the Universe with its structure
- It is needed in computer simulations to provide the large-scale structure observed today
- It has a role in the formation of spiral galaxies
Dark Matter dominates over baryonic matter by a factor of 10
Properties of Dark Matter
It doesn’t interact with electromagnetic radiation (light)
It doesn’t emit/radiate thermal energy (heat)
It has a tendency to exists in clusters
It only appears to interact with the gravitational force
It causes ordinary matter, via gravitation, to clump together and move toward it
It has existed since the early Universe
Nature of Dark Matter
It is not baryonic (i.e. ordinary, quark-based matter)
It is not neutrinos
Cold, weakly-interacting massive particle (WIMP)
Perhaps our current understanding of gravitation (General Relativity)
is wrong?
- Modify gravity
- No evidence for this
Dark Energy and the accelerating expansion
Observed extragalactic type supernovae and the redshift of their spectra
The spectra of these very distant (and thus old)
supernovae was redshifted less than expected
Concluded that the rate of expansion must have been
smaller in the past
so the expansion of the Universe must be accelerating
Dark Energy proposed to account for this additional energy
Properties of Dark Energy
It is very homogeneous – even/uniform across all of spacetime
It does not get diluted by expanding space
- It behaves like an intrinsic property of spacetime
- It now dominates the mass-energy content of the Universe
Not very dense: approx. 10-27 kg/m3
- Air has a density of approx. 1.225 kg/m³
- Seems to only be measurable on cosmic scales
It has negative pressure
- A stretched object has negative internal pressure
- Ordinary matter (a positive gravitational mass) produces a contraction force
- Dark energy acts as a ‘negative’ gravitational mass and produces a repulsive force
What is Dark Energy
The value of the energy density of empty space
A constant energy density filling space homogeneously
Einstein added the cosmological constant to his equations of General Relativity to
preserve a static Universe
Einstein’s “biggest blunder” (that turned out not to be a blunder)
Dark Energy in the past
Today
- Dark energy dominates the energy
composition of the Universe
At time of decoupling (CMB)
- The Universe was 103 times smaller than today
- The Dark energy component of the Universe was 109 times smaller than today
- Completely negligible
The fate of the Universe
The Big Crunch – if the rate of expansion slows over time due to gravity dominating a weakening dark energy, the Universe will collapse
Einstein’s Model – if the Universe expands at a constant rate forever due to a balance between gravity and dark energy
The Big Rip – if the rate of expansion continues to increase due to strengthening dark energy, the Universe will tear apart suddenly
Baryon Asymmetry
Ordinary matter dominates over anti-matter
The temperature of the universe
The early universe is extremely hot
- Homogeneous soup of photons, particles and antiparticles
- Photons and matter are in constant flux of particle-antiparticle annihilation and pair creation
- In thermal equilibrium – a balance is reached
As the Universe expands, the temperature drops
- Eventually particle-antiparticle pair creation no
longer energetically possible
- All the remaining particle-antiparticles annihilate,
leaving only photons
Why isn’t the Universe pure radiation with no matter?
There must have been a tiny imbalance of particles over anti-particles
- We know that the early universe was radiation-dominated
- this means that at the time of the final annihilations, there must have been the following ratio
- Those 10^10 particles and anti-particles annihilated, leaving 1 left-over particle
- Correctly predicts the ratio of H and He in the Universe
If there was perfect symmetry, there would be no matter left over, and the Universe would be pure radiation! What is the cause of this imbalance?
What happened during the Inflationary period?
There was a sudden expansion of the universe soon after the Big Bang
t ̴10^-36 seconds after the Big Bang
- Transition in the fundamental forces
The Universe undergoes sudden, rapid, exponential expansion
- Large vacuum energy creates enormous cosmic repulsion
- Like an extremely strong version of the dark energy that is accelerating space today
- The scale of universe increases by a factor of 10^30
- The observable universe grows from the planck scale to cm scale
- This period of inflation lasts a short 10^-32 seconds
- Then it suddenly stops
Inflation explains observed properties of the Universe
The uniformity of the CMB
The ‘flat’ geometry of the Universe on a large-scale
It also predicts that the real (but unobservable) universe is 10^20 times larger than the observable universe
A fine-tuned Universe
The fundamental properties of the Universe appear fine-tuned, in the sense that they’re just right for the Universe to exist, for stars to form and for life to evolve
- Any slight change and the Universe would collapse
- Fine-tuned to extreme precision
Examples of fine-tuning
Gravitation
dark matter
dark energy
Carbon abundance
strong nuclear force
Explanations of the fine-tuned Universe
Multiverse: Multiple Universes all with their own varieties of constants
The Anthropic principle: Observers are only possible in particular Universes with the right set of constants. Thus it is not remarkable that we are asking these questions
Top-down cosmology: There existed a superposition of possible states in the early universe. Most withered away and only a fraction contributed to the conditions we see today
God?
The Grand Unified Theory (GUT)
The “holy grail” of science – to devise a scientific theory that describes the entire Universe
- Reduce the entire Universe, and of all its interactions, to one, simple equation
4 Fundamental forces
- Quantum Mechanics describes 3 (Strong, Weak and Electromagnetic)
- General Relativity describes 1 (Gravitation)
Paradox of the GUT – any GUT, having ultimate predicative power, would completely determine our search for the GUT
- Incompatible with free will?
