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Plato: on 10/3/08 at 12:16pm UTC, wrote Oops Picture one should be two, and vice versa.

Plato: on 10/3/08 at 12:14pm UTC, wrote One would have to understand the full scope of the geometrical propensity...

Niayesh: on 10/1/08 at 20:41pm UTC, wrote In the beta model, the gas density is assumed to be spherically symmetric...

v: on 10/1/08 at 13:13pm UTC, wrote Niayesh, Not being a physicist, could you explain in simple terms what the...

Zeeya Merali: on 9/30/08 at 19:33pm UTC, wrote Everybody’s been talking about the new and mysterious “dark flow”...


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October 18, 2019

CATEGORY: Blog [back]
TOPIC: From Cosmic Lighthouses to Dark Flow, by Niayesh Afshordi [refresh]
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FQXi Administrator Zeeya Merali wrote on Sep. 30, 2008 @ 19:33 GMT
Everybody’s been talking about the new and mysterious “dark flow” that’s supposedly tugging on galaxies from beyond the edge of the observable universe. Cosmologist Niayesh Afshordi wonders if it really deserves the hype.

From Niayesh Afshordi:

It is no secret that modern cosmologists have developed an obsession with the dark sciences; or at least, that’s what you may think...

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v wrote on Oct. 1, 2008 @ 13:13 GMT

Not being a physicist, could you explain in simple terms what the "beta model" is? what other models are there? and which model do you think might have been more representative? how dramatically would the use of a particular model affect the results?

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Niayesh wrote on Oct. 1, 2008 @ 20:41 GMT
In the beta model, the gas density is assumed to be spherically symmetric and falling off as (1+(r/r_c)^2)^(-1.5*beta), where r is the distance from the cluster center and r_c and beta are constants. The gas temperature is also assumed to be constant. There is no physical reason for any of these assumptions and some of them may be very badly broken. In particular, if you use X-ray data to fit for the best beta-model, it tends to underpredict the gas density on small scales and overpredict it on large scales. The latter could be particularly problematic for interpreting Sunyaev-Zeldovich observations.

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Plato wrote on Oct. 3, 2008 @ 12:14 GMT
One would have to understand the full scope of the geometrical propensity of that event. Whether one might call it the "spherical cow" or not, it follows that this effect blinds, while considered quite a natural function in our everyday world. The light has often caught most of us off guard.

Of course thanks to one of the blog post contributors here should be mentioned to help in this understanding. As well that one would like to help explain PI's understanding and idea of the "mathematical construct" presented by Tegmark

It would be of some help if some more html choices to italicizing quotes might be allowed here and links.

First picture

"We see a pulsar, then, when one of its beams of radiation crosses our line-of-sight. In this way, a pulsar is like a lighthouse. The light from a lighthouse appears to be "pulsing" because it only crosses our line-of-sight once each time it spins. Similarly, a pulsar "pulses" because we see bright flashes every time the star spins."

Second Picture

"When scientists refer to a spherical cow, we are poking fun at ourselves. We are admitting that some of our models or descriptions of things are far more simple than the actual object, like to say that a cow has a spherical shape. The phenomena we study are often complex, and including too many details can hinder, rather than help our understanding. Often it is useful to study a simplified model which contains only the most important general characteristics. Such a model can be more easily studied using numerical or analytical methods and then compared to observations."

It would help to read a bit on spherical cow to get the jest of the understanding what an event can reveal in the Lighthouse.

"Generally the SNR looks different "in each of these different wavelengths", just like you and I look different to another human being (who looks at the visible light) then we do to a bee or a snake (who are able to detect ultraviolet and infrared light, respectively)."

Our measures(In gamma) help us to see in other ways. Consider high energy particle release? Glast. Calorimetric information.


attachments: lighthouse_pulsar.gif, SNRcartoon_small.gif

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Plato wrote on Oct. 3, 2008 @ 12:16 GMT

Picture one should be two, and vice versa.

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