Introduction to Cosmology

Shruti Shrestha
My Introduction
• Master’s degree in Physics from Nepal
Thesis: Cosmology: Black hole with mass and electric charge, but no
spin.

• PhD from Kansas State University from USA

Thesis: Search for Dark Matter particle from Monophoton channel

• Post Doctoral: University of Heidelberg and Fermi National lab

• Teach at Penn State in USA
Introduction
Cosmology Terms:
• Expanding Universe
• Cosmological redshift
• Hubble’s law
Cosmology
Branch of astronomy that studies the origin, structure, evolution of the
Universe.
It seeks to answer some of the most profound questions in science:
• How the universe began (Big Bang)
• What is the universe made of ( Dark Matter, Dark Energy)
• How it has changed over time ( Expansion, Galaxy formation)
• Lastly, what might happen to it in future (Maybe Sun turns into
blackhole and entire solar system will be pulled towards it)
Making Sense of the Universe..
Let’s go back to our solar system.
https://phet.colorado.edu/sims/html/my-solar-system/latest/my-solar-system_all.html

Why does the comet have higher speed when it comes closer to the Sun?
Making Sense of the Universe..
Let’s go back to our solar system.
https://phet.colorado.edu/sims/html/my-solar-system/latest/my-solar-system_all.html

Why does the comet have higher speed when it comes closer to the Sun?
𝐴𝑛𝑔𝑢𝑙𝑎𝑟 𝑚𝑜𝑚𝑒𝑛𝑡𝑢𝑚: 𝐿 = 𝑚𝑣𝑟
Reading Messages From The Cosmos
A telescope collects the light from cosmic objects, we use the light to
extract information about those objects.
When we pass white light through a prism, it disperses into a rainbow of
color that we call a spectrum.
Reading Messages From The Cosmos
Reading Messages From The Cosmos

Common Misconception:
Can you hear Radio Waves?
Reading Messages From The Cosmos
As long as we collect enough light to see the details in spectrum, we can learn properties of the object
we are viewing.
The spectrum of a lightbulb
rainbow of color
Continuous spectrum

The atoms in a warm cloud emit light at

Specific wavelength (color) determined by
the Cloud’s composition and temperature.
Spectrum: Emission lines spectrum
Reading Messages From The Cosmos
As long as we collect enough light to see the details in spectrum, we can learn properties of the object
we are viewing.

𝛽 2.9 ∗ 106 𝑛𝑚 𝐾
𝜆_ max = = = 580 𝑛𝑚
𝑇 5000 𝐾
 Interpreting a Spectrum
Doppler’s effect (Sound) Doppler’s effect (light)
Red shift

https://www.space.fm/astronomy/starsgalaxies/dopplerredshift.html#gallery-2
Interpreting a Spectrum
Doppler’s effect (Sound) Doppler’s effect (light)
Red shift

𝑐 𝑐
= 𝑐/(𝑐 + 𝑣𝑠 ) v = velocity of sound or light
𝜆𝑠ℎ𝑖𝑓𝑡 𝜆𝑟𝑒𝑠𝑡 vs = velocity of source object
𝑣𝑠
1 − 𝜆𝑠ℎ𝑖𝑓𝑡 /𝜆𝑟𝑒𝑠𝑡 = - 𝑐 fo = apparent frequency perceive
𝑐 + 𝑣𝑠 fs = frequency of the sound or light
= 𝜆𝑠ℎ𝑖𝑓𝑡 /𝜆𝑟𝑒𝑠𝑡
𝑐 (𝜆𝑠ℎ𝑖𝑓𝑡 −𝜆𝑟𝑒𝑠𝑡 ) 𝑣𝑠
𝑣𝑠 =
1 + = 𝜆𝑠ℎ𝑖𝑓𝑡 /𝜆𝑟𝑒𝑠𝑡 𝜆𝑟𝑒𝑠𝑡 𝑐
𝑐
Interpreting a Spectrum
Doppler’s effect (Light) Doppler’s effect (light)
Red shift
Interpreting a Spectrum
Doppler’s effect (Light) Doppler’s effect (light)
Blue shift
Interpreting a Spectrum
Doppler’s effect (Light)
Spectral lines provide the reference points we use to identify and measure Doppler shift.
Let’s say, we recognize the pattern of hydrogen lines in the spectrum of a distant object.
Hubble’s Law (Expanding Universe)
Redshift:
Hubble’s Law (Expanding Universe)
The expanding universe: The fabric of space itself is growing
First observed by Edwin Hubble in 1920s when he saw galaxies are receding.
What does it mean?
Space itself is expanding…
Galaxies are not flying through space- they are apart by the expansion of space. (Analogy blueberry
Muffin) rising in the oven.
 Hubble’s Law (Expanding Universe)
• One of the Evidences of Expanding Universe is Redshift of Galaxies:
• Analysis of the light from distant galaxies reveal a clear relationship that the galaxies at greater
distance moving away from us at greater speeds can be expressed by a simple formula.
• Hubble’s law: The recessional velocity of a galaxy is proportional to its distance from the Earth
(Milky Way galaxy).

Hubble’s Law shows:

The further away a galaxy is from the
Earth, the greater the red shift, and the
faster it is moving away.

The closer a galaxy is to the Earth, the

Credit: Katie Moon
smaller the redshift and the slower it is
moving away
 Hubble’s Law (Expanding Universe)
This relationship is Hubble’s law: The recessional velocity of a galaxy is proportional to its distance
from the Earth.

𝐇𝐨 = 𝟕𝟎 𝐤𝐦 𝐬 −𝟏 𝐌𝐩𝐜 −𝟏
Hubble constant (Ho)=69.8 km/s/Mpc,
Researches also reports a high as 74 km/s/Mpc

UChicago astronomer Wendy Freedman

Hubble’s Law (Expanding Universe)
1 light year = 9.5*1015 m
1 parsec ~ 3.3 light years= 3.1*1016 m
1Mega parsec = 1 Mpc =(106) 3.1*1013 km = 3.1*1019 km

A parsec is the distance — 3.3 light-years —

that a star must lie from the Sun for its parallax
angle to be exactly 1 arcsecond (1/3600 of a degree).
Hubble’s Law (Age of the Universe)

If you are 300 km away from house and drove your car 60km/hr, that means 37mph. It will take 5 hrs
to reach home. t = distance/speed

Time it has taken for the galaxies to reach current separation is t = d/v
𝑣 = 𝐻𝑜 𝑑
𝑡 = 1/𝐻𝑜

So, you can take 1/Ho as an estimate for the age of the Universe
Hubble’s Law (Age of the Universe)

𝑣 = 𝐻𝑜 𝑑
𝑡 = 1/𝐻𝑜

Estimation of Hubble constant

ACTIVITY!!
Cosmic Microwave background

https://youtu.be/1kqWWLpyMpY
Hubble’s Law (Age of the Universe)

𝑣 = 𝐻𝑜 𝑑
𝑡 = 1/𝐻𝑜

Reference:
https://www.eso.org/public/spain/images/eso9920o/?lang Estimation of Hubble constant
Hubble’s Law (Age of the Universe)

𝑣 = 𝐻𝑜 𝑑
𝑡 = 1/𝐻𝑜

Reference:
https://www.eso.org/public/spain/images/eso9920o/?lang Estimation of Hubble constant