seven sisters 20051 how i stopped worrying and learned to love the universe marilena loverde...
Post on 19-Dec-2015
213 views
TRANSCRIPT
seven sisters 2005 1
How I Stopped Worrying and Learned to Love the
Universe
Marilena LoVerde
Columbia University
Hubble Deep Field, NASA
seven sisters 2005 2
How I Stopped Worrying and Learned to Love the Universe
• Evidence for Dark Energy
• Models of Dark Energy
• Methods of detecting Dark Energy
seven sisters 2005 3
The Expanding Universe
• Matter Dilutes
• Radiation redshifts
seven sisters 2005 4
Data Sets
Photons decouplet ~ 300,000 yrs Supernova data
7 -13 Billion yrs
NASA/WMAP Science Team
Supernova Cosmology Project LBNL
SDSS
Galaxy surveys1-13 Billion yrs
seven sisters 2005 5
What we find
Flat, homogeneous, and isotropic universe
With accelerating expansion!
seven sisters 2005 6
What could cause accelerated expansion?
Einstein’s Equations + First Law of Thermo.
Fluid equation of state: pressure = w * energy density
has w -1/3 Weird, the energy of this stuff increases with volume, like astretched rubber band.
seven sisters 2005 7
What is this “Dark Energy”?
• Vacuum Energy pressure = - energy density
• Quintessencepressure = w(t) energy density
• Modified gravity ?
• . . .
seven sisters 2005 8
Vacuum Energy?
• Quantum Theory:
QuickTime™ and aNone decompressor
are needed to see this picture.
General Relativity:
Gravity couples toabsolute energy sowe have to includethis
Ground state has a non-zeroenergy
seven sisters 2005 9
Standard Model particle physics predicts:
But we observe:
A disagreement of 1059!!!
Do we expect Vacuum Energy?
Supersymmetric particle physics predicts:
We can calculate this from field theory:
vac ~ MEW4 ~ 1044 eV4
vac ~ MSUSY4 ~ 1048 eV4
vac ~ 10-11 eV4
seven sisters 2005 10
Another problem:
Why now?
matter
radiation
vacuum
We are here
seven sisters 2005 11
QuintessenceV()
Scalar field , potential dominatesover kinetic energy
Quintessence can also cluster
Can solve the coincidence problem
seven sisters 2005 12
Modified Gravity
Extra dimensions?
seven sisters 2005 13
How can we learn more about dark energy?
• From Earth we measure: radiation fluxes, redshifts, and angular sizes of things
flux 1/d2 flux, z dL(z)
size d , z dA(z)
These are exactly what we need to infer theexpansion history
z redshift ~ time
seven sisters 2005 14
Type Ia supernova
European Southern Observatory
Intrinsic luminosity can be determinedfrom light curve:
Saul Perlmutter
Flux luminosity/d2
Standard Candle
seven sisters 2005 15
Baryon Oscillations
Matter Power Spectrum
M. White astro-ph/0507307
d(z)
x
Acoustic oscillations“bump size” can be calculated
“Standard Ruler”
seven sisters 2005 16
Another Probe: Integrated Sachs-Wolfe Effect
In an accelerating universe:grav(t1) grav(t2)
QuickTime™ and aNone decompressor
are needed to see this picture.
seven sisters 2005 17
• Can we distinguish between vacuum energy and quintessence?
• How can experiments be optimized?
• Have we taken everything into account?
My Research
seven sisters 2005 18
Conclusion
• Cosmologists have learned a lot but mostly we have learned we know very little
• Determining the nature of dark energy is of great importance in cosmology and particle physics
• There’s lots to be done to help!
seven sisters 2005 19
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
seven sisters 2005 20
Life of graduate physicists
seven sisters 2005 21
Graduate Life
Typically 2 years of courses and teaching, research first summer and during second year.
~4 years of full-time research, attending seminars, and journal clubs
Also get sent to workshops and summer schools - good fun, free travel
Living stipend (~$22,000/year), basic health insurance and tuition is paid by university for ALL PhD students
seven sisters 2005 22
Is there time for anything else?
Graduate study does take a lot of time more time than a regular job, but your schedule is more flexible. Plus, research, teaching, and learning physics can be very rewarding.
In addition to work I have time and money for dinners, movies, and concerts with friends. Also bit of time to do volunteer work/outreach and I go biking or running daily.
seven sisters 2005 23
Do you want to go?
Most important thing I can say: DO research as an undergrad! Determine whether or not you like it.
Also, attend colloquia, talk to your professors and search the internet to learn about different fields of research
Other options:take time off : full-time research, Peace Corps, Teach for America, Cambridge Part III. .
seven sisters 2005 24
What field?
• Theory or experiment?
• Large or small collaboration?
• Do you mind/want to travel?
seven sisters 2005 25
Applying (I)
Apply to dream schools, realistic schools, and schools that will be very happy to have you.
Find schools that are active in a variety of fields of research
Contact professors and graduate students working on projects that interest you
Apply to fellowships - really great practice writing essays, and fantastic if you get them:
NSF, Hertz, DOE, DoD, NPSC . . .
http://cuinfo.cornell.edu/Student/GRFN/
seven sisters 2005 26
Application (I)
~3 letters of rec, ask 6 weeks early if you can. Provide envelopes, forms, and due date. Ask people who know you beyond your grades in their course. Don’t feel bad about asking, it is part of your professor’s job. Do ask directly if they feel comfortable writing you a good letter.
GRE, practice but don’t worry too much physicists have the highest average score!
Physics GRE, ugh, take practice exams under timed conditions. Dimensional analysis and physical arguments can often eliminate 1-2 answers
seven sisters 2005 27
Application (II)
-Transcripts-Statement of interest: discuss previous research, what
you did, what you learned from the experience. Start and end are the most important parts - make them strong. If you are interested in a specific group say so. Schools are interested in students who are able and likely to complete the program, demonstrate this in your essay. Also discuss your contributions to the community, experiences that solidified your desire to become a scientist
seven sisters 2005 28
Decisions (I)
Once you’re accepted schools will fund a visit to their department.
Talk to the graduate students, ask them the best and worst things about the department.
Are they happy? Does the department care about them?
What percentage of students pass the qualifying exams?
Is there more emphasis on teaching or research?Are you comfortable with the number of women in the department?
seven sisters 2005 29
Decisions (II)
Pick a school with several professors you are interested in working with
Can you communicate with the professors you are interested in working with?
Will they be able to take a student from your year?
Consider the location, can you imagine yourself being happy and productive living there?
Do you like your potential classmates?
seven sisters 2005 30
Being there
• Coursework is demanding, make friends to discuss your work with
• Know that international students arrive more prepared - don’t feel bad about this!
• Try to make some friends outside of the department so you don’t go nuts
• Attend journal clubs, or suggest one• Initiate interactions potential research groups
seven sisters 2005 31
Women in physics
• Yes, mostly male• You are easy to remember• Can form community with other women
seven sisters 2005 32
….qualifying exams
Most schools have some kind of qualifying exam.
You will be tested on core topics (eg mechanics, E and M, quantum, stat mech.) at an advanced undergraduate or beginning graduate level.
Most schools give you two chances to pass. Beware of schools that fail a fixed percentage of students.
Quals are a pain, but think of it as a chance to solidify your knowledge, after all the next time you see the material you may be teaching it!
seven sisters 2005 33
Reading
• First Three Minutes by Steven Weinberg• Introduction to Modern Cosmology by Liddle• wikipedia.org surprisingly good physics
reference• Modern Cosmology by Scott Dodelson• Early Universe by Kolb and Turner• xxx.lanl.gov - free archive of scientific
publications, often very technical• Email me! [email protected]