
Oddball Galaxy Puts Dark Matter Theory to the Test - 0xbxd
https://www.scientificamerican.com/article/oddball-galaxy-find-puts-dark-matter-theory-to-the-test/
======
joe_the_user
What I remember from the announcement of this galaxy's discovery a while ago
was that it didn't challenge the theory of the existence of dark matter. It
only challenged the theory that dark matter surrounded all galaxies. And here,
it dark matter and ordinary matter are attached to each by solely by the force
of gravity, it seem logical that their correspondence would be standard but
universal.

Indeed, to say just a given galaxy lacks dark matter is inherently to say that
dark matter exists.

~~~
nonbel
>"Indeed, to say just a given galaxy lacks dark matter is inherently to say
that dark matter exists."

It means that some assumption somewhere in the calculation is wrong.

------
abakus
I know the Dark Matter Theory is very successful at explaining lots of
phenomenons of cosmological physics. Nevertheless, it always reminds me of the
Aether Theory.

~~~
dogma1138
There isn’t one dark matter theory there are many candidates none of them are
exactly implausible or impossible.

We can’t account for the mass of the universe so we are looking for it.

There are two options either there is mass that we cannot detect through non-
gravitational related observations or that we don’t understand gravity on a
macro level which also puts relativity at odds.

MOND was pretty much killed last year and it’s now completely off the books so
modifying Newtonian gravity to fit in our understanding of the universe won’t
work, which leaves dark matter which is again a place holder name for one or
more types of matter that are very hard to observe.

It can be wimps or it can be that there are a lot more brown dwarfs, rogue
planets, and other bodily objects that we can’t see.

~~~
Avshalom
I don't really follow astrophysics anymore was there anything specific last
year that happened?

(not that various mond's were ever particularly viable)

~~~
dogma1138
Gravitational waves specifically from the colliding neutron stars which were
observed in “real time” in 2017 which arrived at the same time as the light
emitted from the collision pretty much killed it.

~~~
nonbel
Would it affect your opinion if the light emitted was noticed first by another
project, who then told the gravitational wave people about it, at which point
they found data their algorithms had previously rejected as noise and changed
the classification to be a signal?

~~~
davidcuddeback
It was actually detected by two gravitational wave facilities (Hanford and
Livingston) _before_ the GRB was detected by Fermi. What you're referring to
is that the gravitational wave was in a third facility's blind spot (Virgo).
So yes, they had to dig it out of Virgo's data to triangulate the location of
the signal so that they could confirm the event in optical wavelengths (which
they did). No, that doesn't affect my opinion, because your characterization
is inaccurate. It was in fact detected independently as a GW and a GWB.

~~~
nonbel
>"On 2017 August 17 12:41:06 UTC the Fermi Gamma-ray Burst Monitor (GBM;
Meegan et al. 2009) onboard flight software triggered on, classified, and
localized a GRB. A Gamma-ray Coordinates Network (GCN) Notice (Fermi-GBM 2017)
was issued at 12:41:20 UTC announcing the detection of the GRB, which was
later designated GRB 170817A (von Kienlin et al. 2017). Approximately 6
minutes later, a gravitational-wave candidate (later designated GW170817) was
registered in low latency (Cannon et al. 2012; Messick et al. 2017) based on a
single-detector analysis of the Laser Interferometer Gravitational-wave
Observatory (LIGO) Hanford data. The signal was consistent with a BNS
coalescence with merger time, tc, 12:41:04 UTC, less than 2 s before GRB
170817A. A GCN Notice was issued at 13:08:16 UTC. Single-detector
gravitational-wave triggers had never been disseminated before in low latency.
Given the temporal coincidence with the Fermi-GBM GRB, however, a GCN Circular
was issued at 13:21:42 UTC (LIGO Scientific Collaboration & Virgo
Collaboration et al. 2017a) reporting that a highly significant candidate
event consistent with a BNS coalescence was associated with the time of the
GRB959 ."

[http://iopscience.iop.org/article/10.3847/2041-8213/aa91c9](http://iopscience.iop.org/article/10.3847/2041-8213/aa91c9)

~~~
dogma1138
Both LIGO detectors detected and classified it as GW event at the same time as
the GRB classification then it was confirmed through VIRGO data and localized.

[https://www.ligo.caltech.edu/page/press-release-
gw170817](https://www.ligo.caltech.edu/page/press-release-gw170817)

Gravitational waves are all candidates until they can be confirmed with
external sources.

~~~
nonbel
>"Both LIGO detectors detected and classified it as GW event at the same time
as the GRB classification then it was confirmed through VIRGO data and
localized."

All I can say is that other source clearly describes that the Fermi team
announced detection of a gamma ray burst well beforehand, and that the ligo
team treated the situation as exceptional due to that announcement.

>"Gravitational waves are all candidates until they can be confirmed with
external sources."

My understanding is that there is expected to be no accompanying external
evidence for waves generated by inspiraling black holes.

~~~
davidcuddeback
> _All I can say is that other source clearly describes that the Fermi team
> announced detection of a gamma ray burst well beforehand, and that the ligo
> team treated the situation as exceptional due to that announcement._

I explained this in a reply that I just posted to your other comment. GCN is
basically a mailing list where notices are posted in real-time to coordinate
the observation of transient phenomena. If you dig into the citations from the
paper you quoted, you'll read that the GW data takes on the order of 5 minutes
to process before it can register an event, but it was indeed detected in
real-time by their algorithms (gstlal).

There's nothing in the paper you quoted to support the claim that the LIGO
team adjusted their analysis based on Fermi's announcment to GCN. 6 minutes
isn't "well beforehand" when the GW data takes 30-60 minutes for human vetting
([http://iopscience.iop.org/article/10.1088/0004-637X/748/2/13...](http://iopscience.iop.org/article/10.1088/0004-637X/748/2/136/meta)).

> _My understanding is that there is expected to be no accompanying external
> evidence for waves generated by inspiraling black holes._

Multiple signals is the whole point of multi-messenger astronomy. Each
detection was independent, but having evidence in GW, GRB, and optical
provides multiple lines of evidence, which is a cornerstone of good science.

~~~
nonbel
How do you interpret this sentence:

 _" Single-detector gravitational-wave triggers had never been disseminated
before in low latency. Given the temporal coincidence with the Fermi-GBM GRB,
however, a GCN Circular was issued"_

I interpret it as "Because we read the report from Fermi, we treated this data
in an exceptional way". In which case their signal to noise ratio is going to
be messed up, for the background estimates to be valid they need to treat
everything exactly the same every time. Basically, if they want to do this,
Fermi needs to be distributing false positive GRB reports so they can
incorporate them into the background.

>"Multiple signals is the whole point of multi-messenger astronomy. Each
detection was independent, but having evidence in GW, GRB, and optical
provides multiple lines of evidence, which is a cornerstone of good science."

Ok, but I'm saying it is my understanding that no one expects to detect any
supporting evidence in the case of the black holes. Ie, not that it was
missed. There is "nothing to see":

"These momentous black-hole clashes produced gravitational waves that were
audible to LIGO-Virgo but there was nothing to see."
[https://www.nature.com/collections/gghkrvklfb](https://www.nature.com/collections/gghkrvklfb)

~~~
raattgift
LIGO is sensitive to binary NS (and NS stellar-BH mergers) too. Binary NS
mergers throw off a lot of radiation, even or especially if the result of the
merger is a black hole. In the case of GW170817/GRB170817a, the evidence leads
to the conclusion that there was one event and that it was almost certainly a
binary NS merger.

Because gravitational waves' amplitudes fall off linearly with distance rather
than quadratically, because gravitational waves are so weakly interacting that
they essentially do not scatter and are not absorbed, and because the
environments around binary mergers (and between the binaries and us) can
strongly absorb or scatter light (incl. radio and gamma), there should be lots
of detections which have obscure, dim, or even undetectable optical (also
incl. radio and gamma) signals.

Distinguishing among the similar inspiral chirps for similar binary masses
where the composition of the masses differ is an area of active research. The
strong equivalence principle drives the GW similarity, but whereas binary
NSes's surfaces can contact, BH surfaces (if they exist) are deep inside their
horizons, so (ignoring any matter in the neighbourhood, such as colliding
accretion disks) all we get from binary BH mergers is the ringdown as the hair
from the lumpy merging horizon falls off leaving a much more symmetrical
horizon. A mixed binary can shred the NS, leaving behind a brilliant
structure.

For binary NS mergers the GW events are brief compared to optical events (and
the non-gravtiational radiation can interact with (relatively) nearby gas and
dust as light echos and the like, so the results of those emissions can be
studied for longer). It is much more likely that a GW detection will spur a
search by non-gravitational observatories than the reverse. In any case,
everything is carefully timestamped, and the records can be compared well
after that, including by people who aren't experimentalists. :D

~~~
nonbel
> "LIGO is sensitive to binary NS (and NS stellar-BH mergers) too..."

Sure, and the extra info is appreciated, but here is what I was responding to:

>"Gravitational waves are all candidates until they can be confirmed with
external sources."

I believe this is false. They expect to detect many gravitational waves that
will never be "confirmed with external sources". Namely, those triggered by
black holes, and for the reasons you mention.

~~~
davidcuddeback
I don't think "external sources" necessarily means "multi-messenger." GW
detectors are sensitive to local interference, so we have three of them spread
across the globe with non-overlapping blind spots. A GW must be detected in at
least two of the detectors to be confirmed.

I'm not sure if that's what @dogma1138 meant, but I'm pretty sure that's how
the past GW detections have been confirmed.

~~~
nonbel
I agree, thanks.

