Life in the Universe FS 2019 - Simon Lilly
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Life in the Universe
FS 2019
Simon Lilly
1
What is the aim of this course?
An introduction to the astrophysics that is most relevant to try to
understand the place of Life in the Universe.
It is more about the Universe than about Life, because I am an
astronomer, and not a biologist or a biochemist.
The search for our Origins
“21st Century astronomers are uniquely positioned to study
the evolution of the Universe in order to relate causally
the physical conditions during the Big Bang to the
development of RNA and DNA”
Riccardo Giacconi (1997)
2002 Nobel Prize in (Astro-) Physics
2
Some preliminaries
These slides (in PDF) and any other material will be available (usually before the
lecture, hopefully) at:
www.phys.ethz.ch/lilly/educa1on/2019-life-in-the-universe
Seminars: To get formal credit for the course you must do a seminar presenta@on
(you may be asked about your presenta@on in the oral exam). These seminars will be
in the last few lectures and/or other slots depending on how many we have.
20 minute explora@ons of par@cular topics
These will be coordinated by Dr. Bruno Henriques and Dr. Jorryt MaLhee
• Next week: We’ll handout some suggested topics and get from you a beLer idea
of how many would like to give a seminar
• The week aPer: You’ll meet with Bruno and Jorryt during the normal lecture and
agree on your topics etc.
Contact me (HIT J11.2) simon.lilly@phys.ethz.ch
Bruno Henriques brunohe@phys.ethz.ch
Jorryt MaLhee maLheej@phys.ethz.ch
3
What is Life?
Some recognizable features of living
things:
• Growth and development
• Consumption of nutrients
• The ability to reproduce
• Ability to heal and/or to react to
unforeseen circumstances
• Response to external stimuli
• Ability to “learn from experience”
• The whole is greater than the sum
of its parts
• A hierarchy of functions
4
What is Life? Google Dic*onary is not very helpful: Life [noun]: the condi@on that dis@nguishes animals and plants from inorganic maLer, including the capacity for growth, reproduc@on, func@onal ac@vity and con@nual change preceding death. Wikipedia is not much be
For me, the defining characteristic of Life is simply one of degree.
Living systems are extremely “complex” relative to non-living
systems.
i.e. they are made up of particular “structures”, i.e. ordering of
matter, in which very small changes in that ordering lead to quite
different outcomes of functionality.
6
Terrestrial Life: The 20
amino acids out of which
polypeptide/protein chains
are made
How many ways are there to
arrange n amino acids, each of
which can be one of m types?
n=1 m
n=2 m × m /2
n=3 m × m × m /2
n mn/2
Insulin 2051 = 1066
Haemoglobin 20600 = 10780
10780 is an extremely large number:
it is about 10700 times larger than
the total number of protons,
neutrons and electrons in the
observable Universe which is 1080
7
Important point:
Imperfect replication during reproduction,
plus “selection”, is absolutely required for
the generation of this degree of
complexity.
8
How long does it take to generate a
simple 28x(letter+spaces)
sentence** randomly?
Methinks it is like a weasel
Shakespeare: Hamlet. Act iii. Sc. 2.
** equivalent to just a 31 amino-acid chain
On average we would require
2728 = 1040 attempts
If we did 106 per second, this
would take 1034 seconds, or
1016 times the age of the
Universe!
9
Now introduce a
ridiculously naïve
form of replication:
do not change a letter
once it is correct
Success after only
133 attempts!
10A more realistic
replication…. “mothers
and daughters”.
A trial sentence is
replicated 20 times (once
perfectly and the rest
with one random letter
randomly “mutated” into
another). Then we
choose which of the 20
sentences best matches
the target sentence and
use it as the basis for the
next generation.
Success after 92
generations (each with
20 daughters), i.e. only
1840 trial sentences!
11Finally, “blind” evolution…
Same as before, but
without comparison to a
distant “ideal”
Amazing! We still get
a sensible “complex”
sentence after just
121 generations
Bottom line:
(Imperfect) replication plus
selection allows the
generation as well as the
maintenance of complexity
12If life is about order, what about entropy?
S = kB lnΩ Life is a highly ordered i.e. “low-entropy”
Entropy is a measure of state of matter, so what about the 2nd Law
the disorder of the system of Thermodynamics?
“The second law of thermodynamics states that the total entropy can never
decrease over time for an isolated system, that is, a system in which neither
energy nor matter can enter nor leave. The total entropy can remain constant in
ideal cases where the system is in a steady state (equilibrium), or is undergoing a
reversible process. In all spontaneous processes, the total entropy always
increases and the process is irreversible. The increase in entropy accounts for the
irreversibility of natural processes, and the asymmetry between future and past.”
dQ
∫ dS = ∫ −
T
≥0 Does Life somehow therefore
violate the 2nd Law?
Absolutely not! Consider the particular
case of the entropy of N photons S = 3.602NkB 13Consider the whole system
Deep Space at 3K
A few high energy
photons with low Many low energy
entropy photons with high
entropy
The Sun at
6000 K
Earth at 300 K
Important concept of a “stationary non-equilibrium state” with large heat
reservoirs, steady energy flows and an overall entropy production while
14
Sun, Earth and Space preserving a highly ordered stateIt should therefore be no surprise to find that living systems are associated with
extremely high fluxes of energy through them:
Consider an “energy conversion rate” F as a measure of the energy flux through
something
A star like the Sun converts
In a day, a 100 kg human converts nuclear energy into heat. The
about 2000 (k)cal (= 8 MJ) of low Sun has a luminosity of 4×1026
entropy chemical energy (food) into W and a mass of 2×1030 kg
high entropy waste heat, i.e. on
average F = 2 ×10 −4 W kg −1
Much less than a human! Even
6
8 ×10 J −1 allowing for the fact that the
F= = 1 W kg energy generation in the Sun is
100kg ⋅ 8 ×10 4 s
only in the inner 10% by mass
15In my (astronomical) view there are therefore three necessary
(but not sufficient?) requirements for “Life in the Universe”
1. A diversity of atomic species to enable complexity
2. Environments that are
• reasonably warm so that chemical processes occur and,
• reasonably stable over long periods of time for complexity
to develop through (chemical) replication and (Darwinian)
selection
3. Sources of low entropy energy and sinks for high entropy waste
energy, i.e. heat flow down a temperature gradient. This implies
an absence of thermal equilibrium.
Exploring these three necessary conditions
motivates the material in the rest of the course
16Introduction: (1 week)
• What is Life? → generic requirements for Life
• What is the Universe?
Our planetary system: (3 weeks)
• General properties → formation and evolution of planets
• Origin and evolution of Life on Earth
• Possibilities for Life elsewhere in the Solar System (Mars,
satellites of outer planets)
• What about non-Carbon (+water) Life?
Extrasolar planets (3 weeks)
• Indirect detection of extrasolar planets (since 1995)
• New perspectives on planet formation
• Direct detection of planets and possible biosignatures thereon
• How common is life – the Drake equation
• SETI techniques and considerations
17Stars and the origin of the elements: (2 weeks)
• Stars as the source of energy
• Stars as the source of the chemical elements
• The interesting case of Carbon
Cosmology: (2 weeks)
• The size and age of the Universe
• Baryogenesis and the content of the Universe
• The formation and evolution of galaxies
• Cosmological parameters and anthropic considerations
• The future of the Universe
18Are we going to focus too much on “terrestrial-type” Life** to the
exclusion of other possibilities ?
** CNOH-based life on a rocky planet orbiting around a star
I hope you will get the feeling during the course that, even if
other more exotic Life may be “possible”, the terrestrial-type is
by far the “easiest” (and therefore most natural?) way for the
Universe to produce life, and it is therefore presumably the
most common.
19What is the Universe?
20Solar System is a flattened disk with largely parallel spin
and orbital axes. Age is 4.6 Gyr
99.9% of the mass is in the central star (Sun)
21Abundances (by number) in the Galaxy (e.g. Sun) and in the Earth
Key point: “Planet forma1on” leads to
the concentra1on of what are only 2%
impuri1es in the Universe (by mass) so as
to completely dominate the composi1on
of planets 22Rocky with “atmosphere”
More massive gas giants
23Objects of similar mass
Inner Solar Outer
System: Solar
Rocky System:
Icey
Composition
gradients within the
Solar System ->
unlikely to be purely
gravitational 24Comets and
asteroids are small
bodies left over
from the formation
epoch
25The Sun is a normal star:
lifetime ~ 10 Gyr
Other stars have masses in the
range 0.1 – 100 M¤
Stars are “powered” by nuclear fusion
reac@ons assembling atomic nuclei:
H à 4He à 12C à 16O etc
26Space is empty!
The nearest stars are ~4 light-
yearsStars
awayand constellations
(c.f. 8 light-minutes to the
Sun and 5 light-seconds
radius of Sun)
90% of the baryonic material** in
the Galaxy is in 10-22 of its
volume
** protons, neutrons, electrons
Average density of matter in a
galaxy is of order 1 atom cm-3.
In the Universe, it is of order
1 atom m-3
In stars, planets, humans, etc, of
order 1023 atoms cm-3
27
OrionCool gas clouds in space - the Orion Nebula, a large (and largely obscured) star-
forming region. Even here, the typical densities are of order 103 atoms cm
28-3Disks are uniquitous around young stars
Indeed stars form from (accretion) disks
29Indirect detec@on of planets around
other stars since 1995
Direct detec@on – opening
up search for bio-signatures 30What happens at the end of stellar lives? Elements are formed in stars and re-injected into interstellar space by supernova explosions of more massive stars (> about 10 Msun31)
Origin of the elements:
Abundance paLern reflects the crea@on of the elements in stars, either
through fusion reac@ons (up to 56Fe), or via processes occuring in the last
stages of stellar lives (including supernova explosions)
1H, is most common (~72% by mass)
4He is ~26% by mass
Remainder comprise ~ 2% by mass
solar abundance
ratios, typical of other
stars and gas in the
Galaxy
32Galaxies
Galaxies 33
Nearby spiral galaxy M106 observed with HSTTypes of galaxi:es
Typical spiral galaxies like the Milky Way
contain of order
• 1011 M of stars
• 1010 M of gas
• Star formation rate ~ few M yr-1
Typical spatial scale ~ 30,000 light years
(10 kpc)
34Also, typical elliptical galaxies
contain of order
• 1011 M of old stars
• very little gas
• very little star-formation
35Galaxies are distributed non-uniformly and non-randomly, in
filaments and sheets surrounding rela@vely empty voids,
typically 150 million light-years across
1 billion light-years
36There’s more to the Universe than meets the eye:
Visible component of galaxies is concentrated in central part of a dark
maLer “halo” with ρ(r) ∝r-2
In Universe as a whole, non-interac@ng (non-baryonic) “dark maLer”
dominates over the maLer content: ρDM ~ 5× ρbaryons
Furthermore, only 10% of the baryons are in stars/gas in galaxies
Both baryons and dark maLer are dwarfed by the mysterious dark energy,
which is causing the expansion to accelerate
37Also, most radiation in the Universe is not “star-light” but is in the Cosmic
Microwave Background (CMB) at 3 K
3K CMB
Cool dust
Unobscured
reradiating
starlight
absorbed
starlight
Cosmic Microwave Background
(CMB) has Planck Black Body
spectrum at T= 2.728 ± 0.002 K, to
exquisite precision
• MaLer and radia@on in thermal
equilibrium?
• At 2.73 K ?? The CMB is relic of hot dense phase of Universe
(= Big Bang) about 14 billion years ago
38Our Universe is expanding
• RedshiPs of distant objects
• CMB radia@on as relic radia@on
Interesting implications for Life:
• There was a Big Bang “crea@on event” 13.7 billion years ago
• An effec@ve beginning at a finite @me in the past
• A finite extent of the observable Universe
• Expansion fundamentally causes the Universe to be out of thermal
equilibrium (thermal equilibrium was lost when maLer and radia@on
decoupled early in the expansion when T ~ 300,000 K)
Olber’s paradox etc
• Could we imagine Universes not conducive to Life?
• Are there indeed Universes not conducive to Life?
• Is our own (sen@ent) existence a selec@on effect in a Mul@verse? 39One of the deepest views of the Universe:
1% of the area of full Moon contains a few thousand galaxies
at distances up to a look-back of >90% of age of the Universe
(and about six foreground stars in our own Galaxy).
There are about 1011 galaxies in the observable Universe, i.e.
of order 1022 stars.
Do you find these numbers to be large or small?
Consider each star as a 0.2 mm grain of sand….
• there are 1011 grains in 1 m3
• there are 1022 grains in 10m x 100km x 100km (c.f. Sahara)
40How to imagine the size of the Universe in 6½ easy steps
You and me 2m
× 10,000
A large city 20 km
× 10,000
The distance to the Moon 400,000 km
× 10,000
… to Saturn 2 billion km
× 10,000
... to the nearest stars 4 lyr (light-years)
× 10,000
… to the Center of our Milky 30,000 lyr
Way Galaxy
× 100
… to the nearest large galaxy 2.2 million lyr
(Andromeda Galaxy)
× 10,000
… to the edge of the about 20 billion lyr
observable Universe 41
Size of the UniverseHow to imagine the age of the Universe in 4 easy steps
A few sentences 45 seconds
× 10,000
A week 7 days
× 10,000
Time since ETH founded ~150 years
× 10,000
Time since Homo Erectus 1.5 million years
× 10,000
Time since the Big Bang about 14 billion years
42
Age of the UniverseAstrophysical aspects of Life in the Universe
• Production of energy in stars
• Production of the chemical elements in stars
• The concentration of trace chemical elements into planets
• The interaction of planets with the astronomical environment
• The formation and subsequent star-formation history of galaxies
• The formation and evolution of the Universe as a whole
… but first
• Properties of our own and other Solar Systems and how they
formed including (brief) overview of Life on Earth
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