shadows of black holes - idpasc · 2016. 6. 29. · in fact, the bhs lense each other !multiple...

Kerr Black Hole Shadows Hairy Black Holes Shadows of Kerr Black Holes with scalar hair

Shadows of black holes

Pedro V. P. da Cunha1, 2

1 Departamento de Física da Universidade de Aveiro and CIDMA2 IST, Universidade de Lisboa

in collaboration with: C. Herdeiro, E. Radu, H. Rúnarsson

PhysRevLett.115.211102


What is a BH?

Source: Interstellar

Black Holes (BHs) are predicted in General Relativity (GR).

They are compact states from which light cannot escape.

The event horizon is the no turning point.


Sagittarius A*

Gillessen et al.

Center of the Milky Way→ evidence for a supermassive BH.

Stars orbit an extremely massive object: 4× 106M�.

This massive object is dubbed Sagittarius A*.


Kerr Black Hole

M

~J

Kerr space-time→ describes a rotating BH in vacuum.

It is the most standard BH in astrophysics.

The BH is uniquely determined by M and J = Ma.


What is a BH shadow?

Optical perception of a BH→ silhouette over background light.

This dark silhouette is called a shadow.

It is the fingerprint of the BH event horizon and its geometry.


Shadow of a BH merger

arXiv:1410.7775 [gr-qc]

This is how a BH binary would look like!

Here we have two large shadows.

In fact, the BHs lense each other→ multiple shadows!


Backwards ray-tracing


Numerical integration of light rays yields the shadow.

It is more efficient to evolve the rays backwards in time.

This numerical procedure is called backwards ray-tracing.



Celestial sphere

Reference: arXiv:1410.7775 [gr-qc]

The BH and the observer are placed inside a colored sphere.

BH is in the center of the sphere.

The observer is in the equatorial plane.



Shadows of Kerr BHs

The Kerr shadow is not symmetric due to rotation.

Inside the Einstein ring→ inverted copy of the celestial sphere.



Shadows of Kerr BHs

Adapted K.Thorne.

The edge of the shadow has a recurrence structure.

Photons encircle the black hole an increasing number of times.


Event Horizon Telescope

Source: Doleman et al.

Event Horizon Telescope→ international array of telescopes.

It aims to probe Sgr A* and obtain the first images of a shadow.

New observational templates are required!


Introduction to “hair”

Source: Cinema 4D

Uniqueness theorems→ BHs are determined only by M and J.

This lead to the no-hair conjecture.

“Hair” is a metaphor for extra parameters (no Gauss law).


Scalar fields

Source: Cinema 4D

Scalar fields→ simplest descriptions of matter.

One may search for BH solutions coupled with scalar fields.

However, solutions with “good” matter are very hard to find...


Kerr BHs with scalar hair

Einstein-Klein-Gordon system

S =

∫d4x√−g[

R16πG

−∇νφ∇νφ∗ − µ2φ∗φ

].

Einstein’s gravity minimally coupled to a complex scalar field φ.

A stationary BH solution exists, in equilibrium with φ.Herdeiro & Radu: PhysRevLett.112.221101


Shadows of Hairy BHs

Kerr BH with scalar hair Kerr BH (same M, J)



0

0.5

1

0.6 0.7 0.8 0.9 1

Mµ

w/(mµ)

Boson Stars (q=1)

extremal KBHsSH

m=1

q=0.97

q=0.85

q=1

q=0

Kerr black holes

The shadows of hairy BHs can be very distinct from Kerr’s.

These shadows are smaller!



Kerr BH with scalar hair Kerr BH (same M, J)



One can try a different pattern just for fun!


Computational calculations

Source: Sun Microsystems

Previous image: 4000× 6599 pixels.

Amounts to ∼26 million photon orbits!

1/2 day computation in Blafis super-computer (Aveiro).


The road ahead..

The future work:

build observational templates for the Event Horizon Telescope.

study shadows in detail for different latitude angles.

analyse shadows of BHs with other types of hair.


Acknowledgements

Work is supported by the FCT IDPASC Portugal grand No.PD/BD/114071/2015 and by the Calouste GulbenkianFoundation.

Computations were performed at the Blafis cluster, in AveiroUniversity, and at the Laboratory for Advanced Computing,University of Coimbra.

@Gr v

shadows of black holes - idpasc · 2016. 6. 29. · in fact, the bhs lense each other !multiple...

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