Gravitational waves and cosmology

David J. Weir [he/they] - davidjamesweir

This talk:

Before we begin:

About me

About me

  • Associate Professor in Physics
    and Academy Research Fellow
  • In Helsinki (more or less) since my PhD (2011)
  • Originally from Scotland
  • Work on gravitational waves in the early universe
    + processes which make them

But before I tell you about my research,
I want to take you back to 2003...


  • 2002-2003 University of Edinburgh
    (dropped out)
  • 2003-2011 Imperial College London
    MSci, PhD (eight years in one building!)


  • I had no course choice until my 3rd year
    β‡’ very broad, valuable, base of science knowledge
  • Later, when I could, I focussed on theoretical topics:
    • Quantum optics
    • Particle physics
    • Quantum field theory
  • Took history of science and other topics non-credit
    - and nearly started a PhD in history of science
  • My Masters project (pro gradu) was on wetting
    - last minute change to theoretical particle physics

How I got into physics: summer jobs

  • 2003: Condensed Matter, University of St. Andrews
  • 2004: (bummed around, folk music concerts)
  • 2005: Space Physics, Imperial College London
    working on the Cassini space probe πŸ›°πŸ˜ƒ
  • 2006: Two summer jobs,
    1. Theoretical Physics, Imperial College London
      working on complex networks
    2. Applied Mathematics, Imperial College London
      studying mathematical modelling of evolution
  • 2007: CERN OpenLab (computing department)
    working on volunteer computing/citizen science

Summer jobs, Masters, PhD ...

  • Don't rush your studies
    Supervisors and employers care about you developing into a mature, independent scientist
  • Finding projects: talk to potential supervisors + current students:
    • Is it a nice group to work in?
      (for 3 months? 1 year? 4 years?!)
    • Can you get a summer job?
    • What courses do they suggest?
    • Will someone mentor you?
  • Do try new things and take unexpected turns.

Key takeaways:

  • Listen to:
    • Potential employers
    • Senior students
    • Mentors
    • Supervisors
  • Faster doesn't mean better. Don't rush your studies.
  • Interests + connections you develop will matter.
  • Be broad now, you can be narrow later
    ☞ Don't miss out on your new favourite thing.

About gravitational waves and cosmology

What is a gravitational wave?

- Stretches and squeezes a ring of matter

Sources: Wikimedia; ESA

First evidence: Hulse-Taylor pulsar

Source: Shane L. Larson

Hulse-Taylor pulsar

  • Two neutron stars, of which at least one pulsar
  • Orbital diameter: 3 light seconds
  • Orbital period: 7.75 hours
  • Orbit slowly contracting (3.5 metres per year)
  • Energy must be going somewhere...
    gravitational waves
  • Gravitational wave power output: $7\times 10^{24} \, \mathrm{W}$
    (about 2% of the Sun's EM radiation).
Source: NASA
Source: (CC-BY) Andrea Nguyen on Flickr

LIGO at the Hanford Site

Source: (CC-BY-NC-ND) Prachatai

Two neutron stars merging

Light and gravitational waves from neutron stars

(CC-BY) ApJ 848 L12 (2017)

Neutron star merger and cosmology

  1. Photons arrived 1.7s later, after travelling 100 M ly
    β‡’ gravitational waves travel at the speed of light
  2. Independent measurement of universe's expansion:
    • Luminosity of gravitational waves β†’ distance
    • Telescopes observe host galaxy β†’ velocity

What happened in the early universe? when the universe was optically opaque? to dark matter?

LISA mission

To see what happened right after the Big Bang, need to study longer wavelengths, need to go into space!

  • Three arms (six lasers), 2.5 M km separation
  • Launch 2034

Source: LISA.

LISA's orbit

Thank you!

Next up: Essi Vilhonen

Ask questions anonymously of both of us:

Update: link to new blog post

Check out this blog post about what our summer students did this year.

Answers to questions πŸ‘‡

Thanks for all the questions. Hopefully these slides go some way to answering them...

What made you finally decide on theoretical physics?

It was pragmatic: I didn't really know exactly what I wanted to work on, but I valued having four years of funding rather than three - more time to learn and work on things is always valuable. In different circumstances (such as in Finland, where funding is not fixed so far in advance) I might have chosen differently.

Is there something you regret doing (or not doing) during the years?

Not really: I was very fortunate with how things turned out. The one thing I sort-of regret is not moving to a different university between my Master's and PhD. But then, many people have personal reasons for needing to stay where they are for as long as possible, and so I would never tell anyone that they must move.

What is your typical day like?

At the moment, it's quite full of online meetings. Our group has online coffee breaks twice a day, and I try to make it to some of them to see how people are. Now that we all work from home, it's harder to have 'random' interactions with your colleagues, and so even if having lots of online meetings seems tiring, I believe it is a good way of making sure everyone feels included. Hopefully soon we can get back to working in the office, too...

How can the whole universe fit in a thumb?

This is a good question: the universe has been getting bigger for a very long time! Although we can't observe it directly, we can infer from observations, such as of the cosmic microwave background, what happened in the early moments of the universe. We also know from the abundance of very light elements that they formed in a dense, compact, homogeneous space.

The evidence all points toward the universe expanding throughout its history, even if it seems hard to imagine.

If universe is expanding, does it mean that space-time is expanding and therefore time is also expanding?

If the universe is expanding, then yes, space-time is expanding – in a sense they are the same thing.

As for whether time is expanding, well, relativity tells us that is hard to separate out from the other effects. Away from any black holes or other dense objects (even the Earth, although the difference is very small), a clock in any part of the universe will tick at the same rate. Good luck synchronising the clocks though... signals can still only travel at the speed of light.

How much do cosmology and astrophysics cross over?

They're two sides of the same coin, I think. I would roughly say that astrophysics is generally concerned with individual objects or groups of objects, while cosmology is concerned with the universe as a whole.

Some astrophysical measurements (such as how fast distant galaxies are moving away from us) tell us about the nature of the universe, and the structure of the universe today (what astrophysicists observe) is seeded by early universe cosmology.

How come the gravitational waves were detected earlier than the photons? Is it due to some measurement error or is there some other phenomenon?

This is a good question: I don't know. Probably the process which gave rise to the emission of the photons takes a bit longer. You can read more about this observation here on the LIGO collaboration web pages.

Is the potential energy of the universe zero?

Whenever we have a system which has a potential energy, we can add or subtract a constant value everywhere and still have the same physics – so in a sense, the value of the potential energy can be zero if we define it to be, even if its minimum were lower.

Generally, the universe, like any system, will settle to the minimum of its potential. However, we might be in a local minimum of, for example, the Higgs potential energy – with a lower, global minimum that the universe has not reached.