Black holes andEinstein’s elusive waves

Nils Andersson

School of Mathematics

University of Southampton

na@maths.soton.ac.uk

A very brief history

1915 Einstein publishes his General Theory of Relativity, which predicts the existence of black holes and gravitational waves.

1960s The first black-hole candidate (Cygnus X1) is discovered.Mathematicians are struggling to agree that gravitational waves are real.

Today Most galaxies are thought to harbour gigantic black holes. (This is the conservative explanation!)A network of extremely sensitive detectors are in operation, but we are still waiting for the first direct detection of gravitational waves.

2015 The planned launch of LISA, a space based gravitational-wave antenna. Detection is guaranteed……unless the theory is wrong

Black holes

8G T

A black hole “bends” space-time so much that light cannot escape from it. This makes black holes “impossible” to observe directly.

General Relativity is based on the notion of a curved space-time.

The motion of matter and the geometry of space are related through Einstein’s equations

All we know is gleaned from how they affect their surroundings, from radio jets and matter flows.

NGC6240Black hole mergers are thought to be common.

Chandra recently found a system of two massive black holes starting to merge in NGC 6240.

Gravitational waves

…the most important of Einstein’s predictions that has not yet been verified by direct detection.

Gravity is tidal in nature, so gravitational waves change the distance between freely-moving bodies in empty space.

The waves carry huge energies, but interact very weakly with matter. This makes them difficult to detect…

…but it also means that they probe of some of the most interesting parts of the Universe.

LISA is a joint ESA-NASA mission aimed for launch in 2015. The frequency range is a good match to the timescale (hours) of many astronomical systems.

Cosmological sources will provide very strong signals. Expect to see:

– supermassive black hole binaries in merging galaxy cores

– small bodies that are captured and fall into large black holes.

Massive mergers

1. The massive black holes “sink” to centre of merged system (via dynamical friction)

2. A bound binary is formed

3. Binary orbit tightens due to interactions with stars

4. At a critical separation energy loss becomes dominated by gravitational radiation

5. A rapid coalescence follows

An important probe of the early Universe;

– structure formation scenarios

– growth of supermassive black holes

Cosmology models feature build-up of galaxies from smaller structures. Most galaxies are thought to have undergone merger(s).

LISA will be able to detect massive mergers throughout the Universe.

Three phases of radiation:

1. Inspiral phase: frequency and amplitude increase gradually in time

2. Merger phase: strong field space-time dynamics, spin flips and couplings…

3. Ringdown phase: distortion of rotating black hole

Black hole spectroscopy

– inspiral provides distance, can infer cosmological parameters

– measure individual masses and spins

– compare to nonlinear simulations, probe violent space-time dynamics

– detect normal modes of ringdown to identify final black hole

Mapping spacetimes

The massive black hole in a galaxy core captures a compact object once every million years or so.

These survive all the way to the horizon radiating 100,000 wave cycles during final year of inspiral.

Frequencies sweep and shift slowly as the compact object spirals in, mapping space-time outside the horizon.

The frequency is in the middle of the LISA band, and 1 year long filtering could disentangle signal from noise.

Need very accurate theoretical models tracking the evolution of the signal.

Available observations show only a mass concentration. Gravitational waves are the only radiation actually emitted by black holes. LISA will literally hear these black holes merge. 

Challenges

Astrophysics:

Gravitational wave observations will be complementary to those from standard astronomy.

Need to understand any accompanying signals from black hole mergers, and improve our understanding of formation scenarios and event rates.

General Relativity:

Coherent detection means matching to expected waveform plays key role.

Need accurate simulations of strong-field merger, and model of generic orbits around a rotating black hole.

Data analysis:

Massive black hole coalescence will be visible without filtering, but good fitting is needed to remove signals without contaminating weaker signals also present.

Need to remove signals from all galactic binaries!