Talk:Square kilometre array

Review by J. Lazio

Lead paragraph:

I believe that the science case of the SKA is broad enough that it could be highlighted even in the first paragraph. The SKA will do fundamental astronomy related to the evolution of the Universe, but it will also constrain fundamental physics and conduct astrobiological observations, potentially including the detection of life elsewhere in the Universe. Also, to a native English speaker, "Until the next decade" sounds a bit off; I suggest, "Beginning in the next decade ...."

Radio Waves:

All of the information presented here is correct and true, but it somehow seems as if it could be described even more impressively. I contend that observations at radio wavelengths really gave us the modern view of the Universe---radio astronomy has provided us the cosmic microwave background, the first notion of non-thermal emission processes, quasars, neutron stars and pulsars, masers, extrasolar planets, .... Is there a way that this paragraph could be revisited to make it even more impressive and stirring.

Also, I am uncertain about the level of the average Scholarpedia reader, but it might be useful to have links to some of the terms when they are first described. Thus, does the average reader know what a neutron star is?

Key Science Projects:

I recommend moving this to be the next section. No matter how technically compelling, unless scientifically motivated, the SKA is unlikely to attract significant construction funding or ever get built. Also, I recommend beginning this section, even before plunging into the Key Science Projects, with a description of the hyperfine line of hydrogen as that motivates at least two KSPs. As the text is worded now, the Dark Ages portion reads as if the reader knows what the 1420 MHz line is, then the next section introduces it. I also recommend a link to the SKA Science Case, either through ADS, New Astronomy Reviews, or the SKA Web site.

Finally, I will make a few comments about the individual sections, but I recommend more detail in all. In too many cases, it is written as if the reader already knows the topic. Also, more links could be included. In some cases, there are simulations on the Web or links to images would be helpful.

Dark Ages section:

Consider a link or reference to the ARAA paper by Fan et al. (or is it Carilli et al.) from 2006 or so.

Galaxy Evolution, Large-Scale Structure, and Dark Energy section:

This section is written as if galaxy evolution is an afterthought. At least equal time should be spent discussing a key issue of galaxy evolution, namely how galaxies acquire their gas and turn it into stars. Given that hydrogen is the raw material from which stars form, the SKA should be an important asset in terms of understanding galaxy evolution. Moreover, galaxy evolution was the original motivation for the SKA, predating the discovery of dark energy by about 5 years.

With respect to dark energy, the author might consider a link to the Dark Energy Task Force and its report.

Strong-field Tests of Gravity with Pulsars and Black Holes section:

Nowhere does this section mention that the study of pulsars has already generated two Nobel prizes, one for the indirect detection of gravitational waves!

Cradle of Life/Astrobiology

This section appears to suggest that the SKA will detect line emission from biomolecules *from the surface of a planet*. There's no way that will happen. Rather, the SKA will detect the emission from interstellar molecular clouds. One of the key questions for the SKA is whether it will be able to detect emission from protoplanetary disks.

Technical Description:

Nowhere does this section say explicitly that the SKA will be an interferometer. I suggest adding some initial discussion of what an interferometer is and how it operates. An analogy that I use frequently is that, if I cover up a small portion of an optical telescope's mirror with black tape, the telescope will still function. One can then ask the question of how many "holes" can be put into a telescope yet still have it function. The answer is that an interferometer is essentially a telescope with many holes, but because we track both the phase and amplitude of the waves, we can reconstruct the telescope.

Technical Developments

This section doesn't mention either the EVLA, the ATCA, or the VLBA, even though these are examples of single-pixel feed telescopes. I don't recall whether these are "formal" SKA Pathfinders, but they are clearly examples of existing telescopes that illustrate how the SKA will work, and, in some cases, they are exploring various scientific or technical aspects relevant to the SKA.

In addition to SKADS, both the EC FP7 PrepSKA and U.S. Technology Development Program (TDP) should be discussed. Both are contributing significant resources and are helping approach the SKA as a *system*, rather than as a collection of parts.

Timeline:

I do not believe that the "final" technical design will be selected in 2012. As described earlier in this article, there is considerable technical development occurring. Rather, the construction is slated to start in the middle part of the decade, with technologies being incorporated as they mature.

Also, it is probably best to say that the proposed site in Africa is in South Africa, extending northward into the continent, to indicate that in addition to the two countries named, there could be other nations involved in the siting.

References:

Additional references that should be included are recent papers by Schilizzi et al. on the SKA itself (in an SPIE proceedings) and Dewdney et al. on the technical development of the SKA (in a special issue of an IEEE proceedings).

Review by J. Tarter

Radio Waves

The statement ‘they do not suffer from distortions in the atmosphere’ is not correct as a general statement about ‘radio waves’. There is the equivalent of radio ‘seeing’ caused by fluctuations in the dielectric constant of the H2O vapor associated with atmospheric density fluctuations. This is a greater problem at shorter mm wavelengths than at the cm wavelengths of the SKA, but the statement as made covers all ‘radio waves’.

Key Science Projects

I would suggest the following changes for grammatical reasons. “The large investment in the SKA requires convincing justification. Apart from the expected technological spin-offs, five main science questions (“key science”) drive the requirements for the SKA………..”

- Probing the dark ages

Remove “Fortunately” from start of second sentence.

- Galaxy evolution, cosmology, and dark energy

Last sentence in paragraph 1: change “As long as” to “Since” Last sentence in paragraph 2: insert “if” between “or” and “a new idea” Second sentence in paragraph 3: remove “Fortunately” from start of sentence.

- Tests of general relativity and detection of gravitational waves with pulsars and black holes

I would suggest changing phrasing of first sentence to: “The radio astronomical discovery of pulsars and the indirect detection of gravitational waves from a pulsar-star binary system were rewarded with Nobel prizes for physics.” First sentence in paragraph 2: insert “millisecond” between “network of” and “pulsars”

- Origin and evolution of cosmic magnetism

Third sentence in paragraph 1: replace “It seems” with “Data suggest”

- The Cradle of life – no capitalization of cradle

Last sentence in paragraph 1: replace “similar to” with “by spacings of order”

Technical Design

In the first paragraph, please add the Allen Telescope Array to the list of present-day interferometers . Last sentence in paragraph 1: change “only allow” to “demand” First sentence in paragraph 4: I would suggest rephrasing this as follows- “To meet these ambitious specifications and keep the cost to a level the international community can support, planning and construction of the SKA requires many technological innovations such as light, low-cost antennas, detector arrays…….”

Item2. in list of items under investigation for antenna design - until the US TDP completes its analysis of the cost of parabolic antennas vs. diameter, I would suggest replacing the statement “12-15 meters diameter” with “~12 meter diameter”.

Technical Developments

In paragraph 5: the URL for the PHAD web site is missing a “.” after “www” In paragraph 6: please add “ATA” after “LWA” in the list of pathfinder telescopes.

Timeline

I think it would be informative to add “with a core in the Karoo desert” after Southern Africa.