Stars, Galaxies, Universe Final

equivalence principle

Being in free fall is equivalent to being in 0 gravity space

Being in accelerating frame feels like gravity and can't be distinguished

understanding free falling satellites

All satellites in orbit around Earth are in free fall

Combo of speed at given moment in time and acceleration makes them move in circle

SPACETIME and gravity

Space and time are part of spacetime

3 dimensions of space, 1 dimension of time

Motions of objects as function of time

Objects are moving through spacetime

light's path bends due to gravity

Einstein realized this should happen

Bending of light predicted by Einstein is twice that predicted by Newton

Gravity is curvature in spacetime, not force

motion of mercury

Mercury has eccentric orbit (0.21) and orientation of major axis changes slightly over time

Partly expected due to gravitational preturbations from jupiter and saturn

Offset of 40 arcseconds no one could explain, maybe planet close to Sun (vulcan)

Einstein predicted offset of 43 arcseconds per century

deflection of starlight

Eddington read paper by Einstein

Looked for effect during eclipse in South Africa and Brazil in 1919

Einstein's predictions were true to 20% precision

gravitational lensing

Can cause objects to appear in different position, look brighter, or both

spacetime and GRAVITY

Stronger gravity = slower pace of time

We live in low-gravity environment

surface gravity on surface of neutron stars

100 lbs on Earth is 300 trillion pounds on Neutron star

Escape velocity is half of speed of light

limits of fighting gravity

Earth held up by fact that it's hard to squeeze rocks

Sun held up by pressure from lots of nuclear explosions

White dwarfs held up by fact that it's hard to squeeze electrons

Neutron stars held up by fact that it's hard to squeeze neutrons

No known force can fight black holes

event horizon of black hole

Schwarzchild calculated radius from which light can't escape a point mass in general relativity (Schwarzchild radius/event horizon)

Radius proportional to mass of black hole

falling into black hole

Current physical theories allow us to calculate how things behave up to singularity

Observer falling into black hole sees rest of universe blueshift as he approaches event horizon

If he sends signals once per second, they appear to take longer and longer to outside observers

For massive black holes, spaghettification happens inside black hole as tidal forces are shallower at surface

surface gravity of black holes

More massive black holes have smaller surface gravity

Radius of event horizon grows with mass, but gravity grows with radius

Billion solar mass black hole has 1500 times more gravity than Earth

evidence for black holes

We see motion around black holes, accretion on black holes, and gravitational waves

accretion onto black holes

Gas that falls into black hole gets accelerated to very high speeds

If a little energy escapes as light, it's more efficient engine to make light than star

Luminosities of super massive black holes near centers of most large galaxies can reach 1 trillion solar luminosities

mapping milky way galaxy

Hard to map Milky Way Galaxy because we live inside it

milky way galaxy has disk

Herschels were first astronomers to state Milky Way is shaped as disk

Sun is never in middle because there was the same number of stars in every direction

That's not true, it's only because of interstellar dust dimming stars

harlow shapley and milky way cartography

Shapley estimated distances to 93 globular clusters using period-luminosity relations of variable stars

Spherical distribution with center not near Sun but within disk and toward constellation sagittarius

disks and haloes

Halo is said to contain almost entirely old, metal-poor stars that formed with galaxy

Disk contains every kind of star making it younger and more metal-rich

Milky Way's Disk contains bar in central regions

spiral structure

Found in radio observations

We see gas located in specific lines of sight

Sun located in Orion spur

spiral arms

Gas clouds have differential rotation

Parts furthest from center of galaxy move at different speeds than parts close to center

Arms form their own self-gravity

keplerian estimate for galaxy’s mass

• Sun takes 225 million years to go around Galaxy and semi-major axis is 27,000 light years

• We get 100 billion solar masses an estimate if we use Newton’s genralization of Kepler’s third law

• Not quite right as Sun’s orbit isn’t quite and ellipse and there’s isn’t one point mass at center of galaxy, but rather a disk of mass with distribution of mass

orbital speeds in galaxy vs. solar system

• Galactic orbital speeds constantly at large radii

• Solar system orbital speeds decline at large radii

missing mass

• Distant objects orbit galaxy really fast in spite of there being very few visible objects out there

• Either our theory is wrong or dark matter is out there

modified gravity vs dark matter

• Modified gravity models vary, but common model is that at very low accelerations, gravity is stronger than expected

• Dark matter models also vary but common model is that there’s another particle, undetected by current particle accelerates that doesn’t interact much with protons and electrons but accounts for most of universe matter mass

lots of stuff near center of galaxy

• We see a ton of old stars near black hole, a hundredish OB stars indicating recent star formation but no ongoing star formation

Sagittarius A*

• Lots of mass which we can infer from stars moving rapidly

• We can’t see the mass, stars orbit around nothing

• Supermassive black hole with mass of 4 million solar masses

• One star gets as close as 124 AU from black hole

how do supermassive black holes form

• Nearly every massive galaxy has one at center

• When massive stars dies, they form black holes of 20-30 solar masses

• Having these reach 4 million or 4 billion solar masses, either by accreting hydrogen or having merger trees doesn’t seem to work mathematically

thin disk vs halo orbits

• Stars in thin disk have circular orbits 

• Stars in halo have random and elliptical orbits

two kinds of stars

• Discovered by Walter Baade during WWII

• Blackout of Los Angeles during war to make it harder for enemy submarines made the skies darker (good for astronomy) 

• Population I for bright blue stars in spiral arms

• Population II for stars in Halos and globular clusters

Pop I and Pop II Stars

• Pop I stars found only in disk and in nearly circular orbits 

• Bright supergiants, bright main-sequence stars, stars in spiral arms, and stars of young open clusters 

• Pop II stars found everywhere in galaxy 

• Both circular and eccentric orbits

• RR lyrae stars and globular cluster stars

• Pop II stars formed first 

• Pop II stars are 10-14 billion years old and Pop I are less than 12 billion years old

• Pop I stars have have 1-4% of their mass in elements heavier than hydrogen and helium 

• Pop II stars have less, sometimes their 99.999% hydrogen and helium

• All stars in Milky Way are Pop I stars

Why Younger Stars Have More Metals

• Universe stars with 75% hydrogen, 25% helium, and tiny amount of other elements

• Overtime, more heavy elements (metals) are contributed to interstellar gas from supernovae

And other dying stars

• That’s why Sun has 1.5-2% heavy metals

• Stars form heavy elements in interiors through nuclear fusion 

• When stars die, heavy elements released to interstellar medium

• New stars form from enriched gas

• Older stars have fewer metals

protogalactic cloud and monolithic collapse model

galactic mergers occur over time

• Galaxies merge with random orientations which can add shape and structure to galaxy

• Sagittarius dwarf spheroidal is small galaxy accreted by Milky Way and makes big stream

• Globular cluster M54, probably its nuclear star cluster

expanding universe

• From general relativity, universe is expected to expand or contract

• Einstein added a cosmological constant to keep it static

• Hubble used observations of variable stars in other galaxies in combination of Leavitt law to measure distances and redshifts to measure speeds to show distant galaxies are moving away from us

hubble time

• Big Bang was when all galaxies were close together

• Hubble’s constant says that objects further away move faster

hubble time / age of the universe

• Hubble time is time when galaxy separation was 0

• H0 is rate of expansion and 1/H0 gives age of universe

• Universe is 13.8 billion years old

accelerating expansion

• In 1990s, astronomers used standard candles to measure universe expansion

• Want to measure deceleration of expansion of universe

• They measured acceleration, things were moving faster away from each other over time even though gravity should be pulling them back

• Dark energy is pulling the universe apart and astronomers parameterize it with cosmological constant

future pathways to studying universe

• European Extremely Large Telescope being built in Chile will have 39 m mirror, making it most powerful telescope in the world

• Will directly measure accelerating expansion of universe

expansion of universe

• Space itself is expanding

• Only expansion in voids between galaxy clusters because galaxies have enough gravity that they can be same size

• Wavelength of light in accelerating universe also expands

cosmic tug of war

• Gravity

  • From light, matter, and dark matter, pulls things together

  • With enough gravity, universe expansion will slow down and maybe reverse

• Dark energy

  • Mysterious force pushing things outward

  • Could cause acceleration to be faster and faster

eventual fate of universe

• We don’t know since we don’t understand dark energy

• We expect big chill where we only see the galaxy we’re in, all stars die, only black holes left

critical density of universe

• Density needed to bring universe’s expansion to stop at infinity, then universe will have fixed size

• Universe has density of 30% of this quantity, so universe will expand forever

• Combined density of matter and dark energy is equal to critical density

ages of distant galaxies

• Astronomers quote ages in terms of redshift 

• Redshift z =1 galaxies is seen from when universe was half its current size 

• Red shift z =2 is a third and so on

history of an idea

• Beginning of universe is implied by equations of general relativity

• Lemaitre described meaning of these equations

• Described universe as beginning with giant primeval atom that broke into pieces that formed seeds for galaxies

primeval fusion

• Physicists today know more about nuclear physics and know that his primeval fission model cant be correct

• Instead it’s a primeval fusion model, big bang nucleosynthesis

elements of big bang

• Gamow and Alpher made us now believe that only hydrogen, 25% helium, and tiny bit of lithium were produced in big bang

• Heavier elements formed later in stars

first few minutes

• Earlier we go in time, the higher the mean temperature, pressure, and density of universe

• Early in universe’s life when it was extremely hot, it was mostly just radiation but at higher energies there might be dark matter

t = 0

• Our theories of physics can almost get us to t=0

• We don’t know if anything came before universe

.01 seconds

• At .01 seconds, universe was a soup of matter and radiation

• Particles always collided and temp was high enough for light to produce electrons and positrons but not protons and neutrons

3 minutes

atomic nuclei form

• Not before, atomic nuclei are disassociated by high energy photons

• Later, energy from radiation is low enough that electrons can bind to protons 

• 300,000 years after big bang, universe no longer ionized

on first few minutes

• The fact that from big bang theory one can derive 25% mass for helium is huge success

cosmological lithium problem

• Factor of 3 error for abundance of lithium

• Universe seems to contain 70% less lithium than expected

deuterium

• Isotope of hydrogen which has proton and neutron in nucleus

cosmic microwave background

• 300,000 years after big bang, density dropped enough for electrons and protons to form stable hydrogen

• Lot less traffic for photons/light and the universe became transparent

• First free light of universe and now it’s cooled to be cosmic microwave background

properties of cosmic microwave background

• Nearly perfect match to predictions

• Mean temp of 2.73 K

• Good confirmation that universe evolved from initial, hot state

cosmic microwave background not perfectly uniform

• Universe couldn’t be since we have galaxies and voids between them

• Density fluctuations when CMB was made, 380,000 years after big bang, was 1 part in 100,000 

triangles on sphere can have 270 degrees inside

• Our universe has flat geometry

• Angles inside triangle add up to 180 degrees and parallel lines meet at infinity

• Universe could’ve actually been slightly spherical or hyperolical

• On saddle/hyperboloid, they’re less than 180 degrees

standard model of cosmology

• If anybody measures a different model than the parameters, they refute it

• Universe maybe a little older than 13.8 billion years and Hubble’s constant may be a little bigger than 67.31 km/s/Mpc

composition of universe

• 5% of all mass and energy in universe is matter we’re familiar with on earth

• Most ordinary matter is hydrogen and helium in interstellar and intergalactic space

• 0.5% of universe critical density of universe is in stars

• Dark matter and energy is 95%

changing understanding of universe

• In 1970s, we suspected that most of matter in universe was invisible but we thought this might be ordinary matter (protons, electrons) not producing electromagnetic radiation

• By 1980s, it was clear that it was dark matter we hadn’t figured out yet

• By 1990s, experiments showed we live in critical density universe and dark energy contributes 70% of what’s required to reach critical density

elusive nature of dark matter

• Extensions of standard model of particle physics to include weakly interacting massive particles are most widely studied model of dark matter but isn’t proven to be true

• We’ve never seen indirect evidence for dark matter at Large Hadron Collider

2 problems about universe

• Density of universe is close to critical density

  • Most values would be too high (universe has quick crunch) or too low (dark energy wins too early and there’s no galaxies)

• Universe is very homogenous

  • How can parts of universe that never interacted with one another only differ by one part in 100,000?

universe being flat

• Within our theories of gravity, flat or positve/negative curvatures of universe are all okay

• Infinite number of positive or negative curvature values that universe could have

• Flat universe could only have 0 curvature and that’s what we live in

inflation

• Small regions come in contact with each other and blow up much larger and outside each other’s horizon distance

• Increase of 10^50 in size in 10^-35 seconds

• Bigger scale difference than that of observable universe in proton

• Even if universe had curvature, rapid increase in size would flatten it

• We still don’t know what causes inflation

anthropic principle

• Universe seems fine-tuned for life

• Density fluctuations in universe are large enough for galaxies to form but small enough so matter doesn’t collapse into black holes

• If nuclear reactions occured more easily, everything would’ve gone to iron in big bang rather than 25% helium and stars would’ve never formed

• If nuclear reactions were harder, pressure support from them would be harder and stars would collapse into white dwarfs, neutron stars, or black holes

standard model of universe

• 4% regular matter, 23% dark matter, 73% dark energy, and period of inflation early on 

• Requires dark matter or modified gravity, dark energy, and inflation

human body composition

• 99% of mass comes from oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus 

• Complex life on Earth requires a lot of elemental abundance and liquid water to act as medium for things to move through

copernican principle

• Nothing special about our place in universe 

• Galileo showed solar system doesn’t orbit Earth

• Sun isn’t special star

• Milky way isn’t special galaxy

• Planets around other stars are common

• We don’t know if complex biochemistry is common in universe or to what extent it evolves

giordano bruno

• Priest of Dominican Order

• Realized and argued that stars were just like Sun, that planets must orbit these Suns, and sentient beings lived on these planets

fermi’s paradox

• There should be aliens but there’s no evidence despite many chances and decades of searching

proposed solutions

• Life isn’t common

• Life is common, but intelligent life isn’t

• Intelligent life is common, but we can’t see their signals

• Intelligent life is common, but doesn’t interfere

• Intelligent life always self destructs

• Government is keeping intelligent life a secret

building blocks of life

• We’ve seen building blocks like water, sugars, alcohol, formaldehyde, and amino acids

• We haven’t seen proteins in space

miller-urey experiment

• UChicago in 1952

• Started with methane, ammonia, and hydrogen

• Added electric spark source as there would be in early Earth and amino acids were produced

• Scientists analyzed vials in 2007 with more advanced techniques and found it was more successful

• We don’t know original atmospheric composition of Earth

habitable zone

• Water on planet is neither vaporized nor frozen

• Too close to star, water is vapor

• Too far from star, water is frozen

• Larger distance for hotter stars

• Stars evolve as they age and typically get hotter so habitable zone will move

• Life can take billions of years to evolve

• Planets themselves can effect temperature like Earth or Venus

role of other system bodies in keeping earth habitable

• Jupiter and Saturn protect Earth from asteroids and comets, without them, Solar System would have many more comets and asteroids

• Moon keeps Earth’s axial tilt stable, without it there would eventually be much more extreme seasonal variations

• Moon extended Earth’s day, without it, the day would be 18 hours

phase diagram of water

• Point at which water can be liquid is function of pressure

• Also function of salt content and other properties

limits on extremophiles

• We’ve seen life function in temps from -25 to 122 C

• High temp organism can survive to due high pressures that keep water liquid at those temps

• We’ve seen life span acid-base range of 0 (acid) to 13 (alkaline)

• High levels of salt are bad, thats why we use it as preservative 

• Some microbes can grow in water ten times saltier than sea water

life on mars

• People thought images of Mars were fake

• Edgar Rice Burroughs wrote “Princess of Mars” which influenced lots of sci-fi stories

• Probably used to have liquid water but no evidence of it today

• Viking landers of 1970s found no organic molecules or organic activity

• Curiosity (2012) confirmed Mars once had habitable environments with water, energy, and raw materials like benzene and propane

• Found alkanes with 12 consecutive carbon atoms

• El Presidente wants people on Mars

life in outer solar system

• Jupiter’s moon Europa and Saturn’s moons Titan and Enceladus are believed to be good candidates for life

• Outside habitable zone of solar system but have tidal heating from nearby planets

case for life on europa

• Salty ocean beneath crust 

• NASA’s $5 billion Europa Clipper will get there in 2031 to search for habitability

• Will measure composition of gaseous plumes coming out of Europa as well as dust and ice grains leaving it

• Measuring magnetic field should inform if it has salty ocean

case for life on titan

• Has lakes and rivers of methane and petrochemicals

• Might harbor different kind of life

• Lack of liquid water and very cold temps (-180 C) mean large biomass is unlikely

dragonfly mission to titan

• Will launch in 2028 and arrive in 2034

• Will use helicopter to travel which works since Titan has thicker atmosphere than Earth (50% more atmospheric pressure) but 85% less surface gravity 

• Will fly 8 km at a time

• Should be able to measure different compositions at different sites

case for life on enceladus

• Has subsurface oceans

• Blasts plumes of water vapor into space which contain lots of minerals and organic compounds

• Hard to go there since it’s very far away and surface gravity is 98.8% weaker than Earth

• Enceladus Orbilander is proposed to launch 2038 and arrive 2045-2051

habitable world observatory

• Proposed launch date of 2041, mirror of 6-8 m, and cost of $11 billion

• Main objective would be to identify at least 25 habitable worlds 

• Use spectroscopy to search for chemical biosignatures in atmospheres including gases like oxygen and methane which would serve as critical evidence for life 

• Would use high sensitivity and resolution capabilities to trace evolution of galaxies and other cosmic structures 

tianlin space telescope

• From china expected to launch earlier than HWO and be less powerful

drake equation

• We know R* and Fp pretty well now and Milky Way forms roughly one star per year and half of them can host planets

• Astronomers rapidly learning frequency of planets as function of star type, planet mass, and planet-star separation 

• Ne (number of planets per solar system with environment suitable for life) and Fi (fraction of suitable planets which life appears) are now unknown but we should be able to measure/limit them in the next 30 years

• Anything about life becoming intelligent, whether intelligent life lasts or choose to communicate is beyond technology over the next few decades

SETI

• Uses network of radio telescopes to search for alien radio signals 

• So far they’ve found nothing