Textbook Notes (367,894)
Canada (161,478)
Physics (160)
PHYS 183 (51)
Tracy Webb (12)
Chapter 22

Chapter 22

4 Pages
126 Views
Unlock Document

Department
Physics
Course
PHYS 183
Professor
Tracy Webb
Semester
Winter

Description
PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013 22: Dark Matter, Dark Energy and the Fate of the Universe 22.1 Unseen Influences in the Cosmos  astronomers don’t know what the universe is made of  have good reason to think the vast majority of the universe isn’t composed of atoms, but rather dark matter & dark energy  nature of dark matter & dark energy is unknown  dark matter: a name given to whatever unseen influence is causing observed gravitational effects  dark energy: a name given to whatever unseen influence is causing the universe expansion to accelerate 22.2 Evidence for Dark Matter  evidence for dark matter in galaxies? o distribution of mass in Milky Way  use orbital motion of a star to measure mass of Milky Way within star’s orbit  could do the same thing with orbits of stars at every distance from galactic centre  velocity hard to determine due to interstellar dust obscuring view  but can use Doppler shifts & radio lines with geometry  rotation curve: plots the rotational velocity of stars or gas clouds against distance from centre of galaxy & summarizes results of orbital velocity measurements  rotation curve of solar system drops off with distance from sun  inner planets orbit at faster speeds than outer planets  most mass in Milky Way not concentrated at centre  orbits of progressively more distant gas clouds encircle more & more mass  most of the mass lies within a spherical halo surrounding disk of galaxy  very little radiation detected from enormous mass, so qualifies as dark matter o dark matter in other spiral galaxies  determine amount of dark matter in galaxy by comparing its mass and luminosity  use luminosity to estimate amount of mass from stars  determine total mass of galaxy by applying law of gravity to observations of orbital velocities of stars & gas clouds  total mass larger than mass attributed to stars, excess mass must be dark matter  determine luminosity through telescope & inverse square law for light  determine total mass through orbital speeds of stars/gas clouds & Doppler shifts  combine observations to construct rotation curve o dark matter in elliptical galaxies  weigh inner parts by observing motions of stars which are disorganized, so no rotation curve  velocity of each star still depends on mass  compare spectral lines of different regions & find speeds of stars remain constant as we look farther form galaxy’s centre  most matter lies beyond distance where light trails off & so must be dark matter  what is the evidence for dark matter in clusters of galaxies? o orbits of galaxies in clusters PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013  calculate speeds of galaxies moving away from us through redshifts & determined the recession speed  subtract this speed from each galaxy’s speed to determine speeds of galaxies relative to cluster centre  only tells average radial component, speed toward/away from us  averaged them to get average orbital velocity to determine mass & and compared it to its luminosity  found clusters of galaxies have greater masses than luminosities suggest  most matter mustn’t be in form of stars but almost entirely dark o hot gas in clusters  observe xrays from hot gas that fills space between galaxies  tells about dark matter since temperature depends on total mass of cluster  temperature reflects average kinetic energies, so use xrays to determine particle speeds & the cluster’s total mass  gravity of dark matter seems to be binding galaxies of cluster together in same way gravity helps bind individual galaxies together o gravitational lensing  gravitational lensing: masses distort spacetime, so massive objects bend light beams passing nearby  light bending angle depends on mass of object doing the bending, so measure masses by observing how strongly they distort light paths  evidence of dark matter relies on understanding of gravity  seems following must be true: o dark matter really exists & we are observing effects of its gravitational attraction o there is something wrong with our understanding of gravity that is causing us to mistakenly infer the existence of dark matt
More Less

Related notes for PHYS 183

Log In


OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit