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How I Stopped Worrying and Learned to Love the

Universe

Marilena LoVerde

Columbia University

Hubble Deep Field, NASA

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How I Stopped Worrying and Learned to Love the Universe

• Evidence for Dark Energy

• Models of Dark Energy

• Methods of detecting Dark Energy

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The Expanding Universe

• Matter Dilutes

• Radiation redshifts

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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

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What we find

Flat, homogeneous, and isotropic universe

With accelerating expansion!

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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.

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What is this “Dark Energy”?

• Vacuum Energy pressure = - energy density

• Quintessencepressure = w(t) energy density

• Modified gravity ?

• . . .

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Vacuum Energy?

• Quantum Theory:

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General Relativity:

Gravity couples toabsolute energy sowe have to includethis

Ground state has a non-zeroenergy

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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

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Another problem:

Why now?

matter

radiation

vacuum

We are here

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QuintessenceV()

Scalar field , potential dominatesover kinetic energy

Quintessence can also cluster

Can solve the coincidence problem

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Modified Gravity

Extra dimensions?

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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

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Type Ia supernova

European Southern Observatory

Intrinsic luminosity can be determinedfrom light curve:

Saul Perlmutter

Flux luminosity/d2

Standard Candle

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Baryon Oscillations

Matter Power Spectrum

M. White astro-ph/0507307

d(z)

x

Acoustic oscillations“bump size” can be calculated

“Standard Ruler”

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Another Probe: Integrated Sachs-Wolfe Effect

In an accelerating universe:grav(t1) grav(t2)

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• Can we distinguish between vacuum energy and quintessence?

• How can experiments be optimized?

• Have we taken everything into account?

My Research

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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!

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Life of graduate physicists

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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

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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.

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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. .

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What field?

• Theory or experiment?

• Large or small collaboration?

• Do you mind/want to travel?

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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/

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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

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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

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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?

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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?

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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

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Women in physics

• Yes, mostly male• You are easy to remember• Can form community with other women

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….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!

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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! marilena@phys.columbia.edu